Luminous Landscape Forum

Equipment & Techniques => Landscape & Nature Photography => Topic started by: macgyver on May 16, 2006, 11:35:18 PM

Title: Sensor size and DoF
Post by: macgyver on May 16, 2006, 11:35:18 PM
I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc.  But I don't know why.  Could I persuade anyone to enlighten me, please?
Title: Sensor size and DoF
Post by: Sheldon N on May 17, 2006, 01:24:11 AM
While there's a lot of optical theory involved in an exact answer, most of which is over my head (things such as circle of confusion, degree of enlargement, etc), the primary reason is relatively simple.

To get the equivalent field of view and image framing with a larger format camera, you must use a longer lens at the same focus distance. Think of it this way, if you're shooting 35mm with a 200mm lens at 8 feet distance, your depth of field is very shallow, even at f/8. Now a large format shooter has to do the same thing (200mm lens, 8 feet) just to shoot a waist up portrait. They're still stuck with the depth of field of a 200mm lens, focused at 8 feet and f/8, even though the field of view is closer to a 55mm lens (in 35mm format).

Hope this helps!

Sheldon
Title: Sensor size and DoF
Post by: Anon E. Mouse on May 17, 2006, 03:09:55 AM
DOF is proportional to magnification. Not just at the sensor, but also in relation to display size. When looking at an 8x10 print, which image was enlarged more, an APS size one, or a full-frame 35mm one? So when using the same focal length, the APS image is being enlarged more to reach the same display size and suffers a slight loss of DOF - notice, the angle of view is not the same. (If the display sizes are changed in relation to the different sensor sizes then you are back to the same DOF.)

However, when at the same angle of view, which means different focal lengths and image plane magnifications, the smaller sensor has greater depth of field.
Title: Sensor size and DoF
Post by: Anon E. Mouse on May 17, 2006, 03:39:12 AM
BTW, this assumes the same viewing distance. DOF increases with viewing distance of the display as well.
Title: Sensor size and DoF
Post by: Sheldon N on May 17, 2006, 10:43:56 AM
Anon E Mouse is right that the issue of degree of enlargement affects depth of field. It is a little confusing in that the principle works in what you would think is the wrong direction. For example, small formats have to be enlarged more to reach a given print size, so they in theory would have less depth of field (more enlargement = less depth of field).

However, having to use a longer focal length lens to achieve an equivalent angular field of view has a much larger effect, so the net result is that larger formats have functionally less depth of field at comparable angular fields of view, focus distance, and aperture.

For a sense of the scale of the issue, here are some rough guidelines to think about in terms of depth of field.

A 1.6 crop camera has roughly 1 1/3 stop more depth of field than a full frame camera. (ie. f/1.8 on 20D with 50mm lens = ~f/2.8 on 5D with 80mm lens).

A 35mm full frame camera has about 4 1/2 stops more depth of field than a 4x5 camera. (ie. f/1.4 on 5D with 35mm lens = ~f/6.4 on 4x5 with 135mm lens)
Title: Sensor size and DoF
Post by: gmitchel on May 27, 2006, 06:19:36 PM
This conversation suggests that a less-than full frame sensor increases subject magnification and affects DOF. It does not.

It is a misnomer to refer to the effect of a smaller sensor as a multiplier, a multiplication effect, etc. Camera manufacturers label it that way, and that is not at all what happens. It confuses people.

A 100mm lens still has a 100mm focal length, whether it is used on a full frame sensor or not. The sensor size merely affects the field of view, not the subject magnification.

I have used a Canon 180mm "L" macro lens for years. Whether I use it on my 1Ds MkII or my Canon 20D, I get exactly the same size subject if I use a tripod and keep the subject distance fixed. Since the subject distance and subject magnification are unaffected, so is the DOF. All of this is very easy to demonstrate.

What will be different is that the 1Ds MkII image will show a wider FOV. The FOV when the same lens is used on a 20D is much narrower. It is approximately the same as if the lens was 270mm. This is often a benefit for macro shots because it makes it easier to separate the subject from the background.

When you use a smaller sensor but keep the focal length the same with a 35mm lens, the smaller sensor captures a smaller part of the image from the lens. The lens is still designed (and used by the camera) to create an image the size required for 35mm film. You just get the central portion of that image with a smaller sensor. The rest falls outside the sensor. The portion you do get is identical to what you get with a full-frame sensor.

Cheers,

Mitch
Title: Sensor size and DoF
Post by: Doug Kerr on May 27, 2006, 11:40:14 PM
Quote
I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc.  But I don't know why.  Could I persuade anyone to enlighten me, please?
[a href=\"index.php?act=findpost&pid=65745\"][{POST_SNAPBACK}][/a]
If we try to compare depth of field performance on two different cameras, we need to adopt some premises for comparison.

Here is one set that is often used:

1. We use focal lengths on the two cameras that yield the same field of view.

2. We have the subject at the same distance.

3. We use an apertureof  the same f/number.

4. We use, as our criterion of acceptable blurring, circle of confusion diameter limit (COCDL) values that are the same fraction of the camera's format size.

Note that condition 4 is consistent with saying that we will judge blurring on the same size print, viewed from the same distance, for both cameras.

If we adopt those premises for comparison, then the camera with the larger format will have less depth of field.

As to why this is, it is easiest to say "it comes form the depth of field equations". The parameters that are different between the two situations are:

A. The focal length is greater on the larger-format camera (per item 1).  That greatly reduces the depth of field (it goes roughly as the inverse of the square of the focal length). The reason has to do with longitudinal magnfication and the relationship between depth of focus and depth of field, but I won't bore you with that right now.

B. Our tolerance for the size of the blur figure is increased (per item 4). That of course leads to an increased depth of field, but not to the degree required to overcome item A (it goes roughly as the COCDL).
Title: Sensor size and DoF
Post by: gmitchel on May 28, 2006, 04:55:05 PM
Quote
If we try to compare depth of field performance on two different cameras, we need to adopt some premises for comparison.

Here is one set that is often used:

1. We use focal lengths on the two cameras that yield the same field of view.

2. We have the subject at the same distance.

3. We use an apertureof  the same f/number.

4. We use, as our criterion of acceptable blurring, circle of confusion diameter limit (COCDL) values that are the same fraction of the camera's format size.

Note that condition 4 is consistent with saying that we will judge blurring on the same size print, viewed from the same distance, for both cameras.

If we adopt those premises for comparison, then the camera with the larger format will have less depth of field.

As to why this is, it is easiest to say "it comes form the depth of field equations". The parameters that are different between the two situations are:

A. The focal length is greater on the larger-format camera (per item 1).  That greatly reduces the depth of field (it goes roughly as the inverse of the square of the focal length). The reason has to do with longitudinal magnfication and the relationship between depth of focus and depth of field, but I won't bore you with that right now.

B. Our tolerance for the size of the blur figure is increased (per item 4). That of course leads to an increased depth of field, but not to the degree required to overcome item A (it goes roughly as the COCDL).
[a href=\"index.php?act=findpost&pid=66759\"][{POST_SNAPBACK}][/a]

Let's not confuse our discussion with unnecesary talk about the circle of confusion.  

The circle of confusion is the same when both cameras are 35mm DSLRs. Their sensors are positioned the same distance from the lens. The lens has the same focal length, etc.

With a less than full-size sensor, you are getting a smaller central portion of that same circle of confusion. That's all.

Imagine that you took 35mm film and painted an opaque back ink frame around the film so that only the central 2/3 of the film frame is exposed to the light during exposure. Do you believe you get subject magnification or affect DOF? Well, you wouldn't.  Ask anyone who's had the film transport in a 35mm film camera go screwy.

A less than full-size sensor has no sensors around the periphery of a 35mm frame, just as if you blocked the extra sensors of a full-frame sensor with something opaque. That's the only difference in a 35mm DSLR. All you get is reduced FOV. You do not get a boost in magnification, subject or otherwise.  

Cheers,

Mitch
Title: Sensor size and DoF
Post by: Sheldon N on May 28, 2006, 05:49:00 PM
Mitch -

You cannot separate the issue of depth of field from the concept of a circle of confusion. Depth of field is a construct that is defined by the application of a chosen circle of confusion. You change the arbitrary number that is the circle of confusion, and you change the depth of field. Talking about one without the other is like trying to write in English without using the alphabet.

Fortunately, we have standarized on a circle of confusion that represents the average person's ability to see detail in an 8x10 print at normal viewing distances. Change the print size, viewing distance, or the degree to which you have to enlarge your "negative" to get to an 8x10 print, and you have to change the circle of confusion, which changes the depth of field. Of course, it changes no true optical properties of the lens/camera/film, it just changes what we perceive to be in focus.

You are correct that sensor/negative size has nothing to do with "magnification" as though a smaller sensor was a magic teleconverter. However, it does have everything to do with enlargement. The more you have to enlarge a negative to get to a standard print size, the more stringent a circle of confusion you have to use on the negative to have the standard print be acceptably sharp (acceptable DOF). In the context of this parameter, smaller formats actually have less depth of field, because they must be enlarged more.

However, this is overwhelmingly outweighed by the key issue (which is the answer to the original question). When you use a smaller format, you tend to use a shorter focal length lens and stand farther away with that lens than you would with a larger format.

Shorter focal length lens = more depth of field.
Standing farther away = more depth of field.

I hate DOF threads, why do I let myself get sucked in?  
Title: Sensor size and DoF
Post by: gmitchel on May 28, 2006, 06:36:59 PM
Quote
Mitch -

You are correct that sensor/negative size has nothing to do with "magnification" as though a smaller sensor was a magic teleconverter. However, it does have everything to do with enlargement. The more you have to enlarge a negative to get to a standard print size, the more stringent a circle of confusion you have to use on the negative to have the standard print be acceptably sharp (acceptable DOF). In the context of this parameter, smaller formats actually have less depth of field, because they must be enlarged more.

However, this is overwhelmingly outweighed by the key issue (which is the answer to the original question). When you use a smaller format, you tend to use a shorter focal length lens and stand farther away with that lens than you would with a larger format. [{POST_SNAPBACK}][/a] (http://index.php?act=findpost&pid=66808\")

That's adding other factors besides the size of the sensor. That shifts the argument from a discussion of sensor size and DOF to sensor size, DOF, and other stuff.

The discussion was about whether an APS-C size sensor in a 35mm DSLR alters DOF or subject magnification. We both agree, it does not.  

Alter anything else, of course things will change.  But that misses the "key" issue in these DOF threads.

If you alter the distance to change the FOV, of course you will alter the DOF. But that has nothing to do with the sensor size. It has everything to do with repositioning the camera. Why confuse the issue?

You cannot get the same subject magnification and FOV with a full-frame sensor and a smaller sensor at the same subject distance. If you want the same subject size, you have to sacrifice FOV for the smaller sensor. Want the same FOV, you have to sacrifice subject magnification.

Same lens and same subject distance, full-frame or not, you get the same reproduction ratio. Use the same enlargement for both images, and nothing changes. Use different enlargements for your images and any differences in image appearance owe to those different enlargement ratios, not the differences in the size of the sensors.

I believe it is helpful, in discussions like this, to focus on what changes and what does not. If we hold everything else constant and change only the sensor size, the only thing that changes is FOV. Subject magnification and DOF are completely unaffected.

I do not tend to stand any further back with my 20D than I do with my 1Ds MkII. The relationship between subject and background and the overall effect I want the image to have on the viewer determine what focal length lens I use and where I stand.

Many people assume they get less DOF with a smaller sensor because of a telephoto multiplier effect. It is that widely held misunderstanding that I have been addressing.

The conversation about Circle of Confusion just adds to the confusion. That is a factor determined by the lens manufacturer for the expected enlargement factor. Medium format film cameras use a lower Circle of Confusion because the expected enlargement ratio is smaller. Same lens on two 35mm DSLRs with the same subject distance and aperture, one full-frame and one not, yields the same Circle of Confusion and the same DOF. You just see less of it with the samller sensor.

I see this argument all over the different forums. With a 10D or a 20D or whatever, you get a 1.6x multiplier effect, so your 300mm lens becomes the equivalent of a 480mm lens. (It does, but in FOV only.) The logic is then extended to DOF. Since the lens is the equivalent of a 480mm lens, DOF must be the same as a 480mm lens. Nope. For subject magnification and DOF, the lens is still the equivalent of 300mm.

Michael Reichmann has a really nice essay on DOF, Circle of confusion, etc.:

[a href=\"http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml]http://www.luminous-landscape.com/tutorial...eries/dof.shtml[/url]

Cheers,

Mitch
Title: Sensor size and DoF
Post by: Sheldon N on May 28, 2006, 07:57:18 PM
I agree with everything you wrote.

The only issue is that to measure DOF, you must choose a circle of confusion. In reality there is no DOF, only focus plane then gradually increasing blur. DOF simply is a measurement of how much blur we will tolerate before something becomes "out of focus". You have to draw the line between "acceptable focus" and "out of focus" somewhere, which is done by picking a circle of confusion. You can pick one, I can pick one, the lens manufacturer can pick one, but it's still an arbitrarily chosen number that's the determining factor of how much "DOF" you have.

I guess I should also clarify my statement on the "standing further back" point. You only stand further back with a smaller format if you hold the lens focal length constant. I should have said that with a smaller format you use a shorter focal length lens OR you stand further back.

Another good article on DOF, more relating to the issue of full frame 35mm vs. 1.6 crop digital:

http://www.photo.net/learn/optics/dofdigital/ (http://www.photo.net/learn/optics/dofdigital/)

I'm more in the mindspace of 4x5 vs DSLR. I was just outside in the front yard trying to do a 1:1 macro shot with 4x5. Talk about murderous DOF issues!
Title: Sensor size and DoF
Post by: gmitchel on May 28, 2006, 09:50:10 PM
Quote
The only issue is that to measure DOF, you must choose a circle of confusion. In reality there is no DOF, only focus plane then gradually increasing blur. DOF simply is a measurement of how much blur we will tolerate before something becomes "out of focus". You have to draw the line between "acceptable focus" and "out of focus" somewhere, which is done by picking a circle of confusion. You can pick one, I can pick one, the lens manufacturer can pick one, but it's still an arbitrarily chosen number that's the determining factor of how much "DOF" you have.
[a href=\"index.php?act=findpost&pid=66821\"][{POST_SNAPBACK}][/a]

To determine the width of the DOF, yes, I agree. But that has nothing whatsoever to do with DOF and sensor size.  

That's why I say, discussion of Circle of Confusion only adds confusion here. It is a constant when we talk about full-frame sensor v. smaller sensor. It's a relevant factor in other contexts. But not when we compare the results we can expect with a full-frame sensor or a smaller sensor using the same lens.

Nice chatting with you.

Cheers,

Mitch
Title: Sensor size and DoF
Post by: Sheldon N on May 29, 2006, 02:27:44 AM
I can agree with that, if you just want to say that something has more DOF or less DOF without indicating what that DOF is, you can toss out the circle of confusion.

Amazing, a DOF thread that ends with agreement! What a great forum this is!

Title: Sensor size and DoF
Post by: Doug Kerr on May 29, 2006, 10:26:57 AM
Hi, Mitch,

Quote
Let's not confuse our discussion with unnecesary talk about the circle of confusion.  [a href=\"index.php?act=findpost&pid=66803\"][{POST_SNAPBACK}][/a]

I was not talking about some circle of confusion, or even the diameter of some circle of confusion. I was speaking of the circle of confusion diameter limit (COCDL) which we adopt as a criterion of acceptable blurring, without which concept there is no such thing as depth of field. (The ambiguity in terminlogy is why I don't like calling this adopted number "the circle of confusion", and I don't call it that.)

Quote
The circle of confusion is the same when both cameras are 35mm DSLRs. Their sensors are positioned the same distance from the lens. The lens has the same focal length, etc.

Would a 35-mm dSLR be one whose sensor size is 36 mm x 24 mm?

Indeed, if we have a camera with a certain focal length lens, focused at a certain distance, with a certain aperture (as an f/number), then for a point on an object at some specific distance other than the focus distance, the diameter of the circle of confusion (its actual diameter, not a limit we place on that diameter as a criterion of "acceptable blurring") will be the same regardless of format size. (The optical system of course "has no idea" how large is the film mask or digital sensor array at the focal plane.)

But maybe that's not what you mean.

Maybe you mean, "for a given format size, many people use the same circle of confusion diameter limit for reckoning depth of field." But hardly always.

Quote
With a less than full-size sensor, you are getting a smaller central portion of that same circle of confusion.

I can't imagine what you mean by that. Are you by any chance confusing "circle of confusion" with "image circle"?

Regarding magnification, (image) magnfication is a function of the focal length and the distance to the subject. I certainly didn't say anything about (image) magnfication. I discussed longitudinal magnfication, which is a different matter althogether. It is involved in relating depth of focus to the corresponding depth of field.

In any case, I'm afraid I'm not able to follow the point you are making.

Best regards,

Doug
Title: Sensor size and DoF
Post by: Jack Flesher on May 29, 2006, 11:22:01 AM
What do I have to lose?  I'll jump in to the fray  

1) DoF is a function of aperture, focal length and subject distance from the camera, or more simply stated: aperture and subject magnification factor.  Note that sensor size has nothing to do with either equation.

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF.  So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

3) CoC diameter is a CHOICE and hence is a variable in the DoF equation. As a result it always generates some confusion when discussed.  Simply stated, for a given PRINT size and resolution desired (photographer's choice and hence variables), CoC will need to change based on sensor size; smaller sensors will require a smaller CoC to generate the same print resolution as the larger sensor, but the change is not proportional.  

3a) IMO the confusion arises because the CoC required does NOT change proprtionately with subject magnification factor, it changes as the inverse square -- which again is why a smaller sensor will generate MORE DoF in a given image when all else is equal.
Title: Sensor size and DoF
Post by: Sheldon N on May 29, 2006, 12:14:27 PM
Quote
What do I have to lose?  I'll jump in to the fray  

1) DoF is a function of aperture, focal length and subject distance from the camera, or more simply stated: aperture and subject magnification factor.  Note that sensor size has nothing to do with either equation.

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF.  So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

3) CoC diameter is a CHOICE and hence is a variable in the DoF equation. As a result it always generates some confusion when discussed.  Simply stated, for a given PRINT size and resolution desired (photographer's choice and hence variables), CoC will need to change based on sensor size; smaller sensors will require a smaller CoC to generate the same print resolution as the larger sensor, but the change is not proportional. 

3a) IMO the confusion arises because the CoC required does NOT change proprtionately with subject magnification factor, it changes as the inverse square -- which again is why a smaller sensor will generate MORE DoF in a given image when all else is equal.
[a href=\"index.php?act=findpost&pid=66850\"][{POST_SNAPBACK}][/a]

Agreed. Except that in #3a you state that the change in the CoC is why the smaller format has more DOF. Actually, the CoC works against the smaller format (opposite direction), but the reasons you stated in #2 completely outweigh that and lead to the smaller format having less DOF.
Title: Sensor size and DoF
Post by: Jack Flesher on May 29, 2006, 05:00:43 PM
Quote
Except that in #3a you state that the change in the CoC is why the smaller format has more DOF. Actually, the CoC works against the smaller format (opposite direction), but the reasons you stated in #2 completely outweigh that and lead to the smaller format having less DOF.
[a href=\"index.php?act=findpost&pid=66855\"][{POST_SNAPBACK}][/a]

You misunderstood -- and I agree the smaller CoC works against the smaller format.  As I said, they did not change proportionately but CoC changes at a slower rate, the inverse square.  As such, the change required for a smaller CoC with the smaller format progresses less than the subject magnification change does to generate the same framing as the larger-sensor image.
Title: Sensor size and DoF
Post by: gmitchel on May 29, 2006, 08:02:40 PM
Quote
[snip]

I was speaking of the circle of confusion diameter limit (COCDL) which we adopt as a criterion of acceptable blurring, without which concept there is no such thing as depth of field.

[snip]

Would a 35-mm dSLR be one whose sensor size is 36 mm x 24 mm?

[snip]

[a href=\"index.php?act=findpost&pid=66847\"][{POST_SNAPBACK}][/a]

A 35mm DSL does not need to have a full frame 35mm sensor. It needs to be able to use optics designed for 35mm film cameras. The Canon D30/D60/10D/20D/etc. are all 35mm DSLRs. They use the same Canon EOS lenses used with 35mm film cameras and full-frame 35mm DLRs like the Canon 1Ds MkII.

The optical system in 35mm DSLRs are designed to create images for 35mm film cameras or 35mm DSLRs. As you say, the lens knows nothing about the medium capturing the image.

Some of the 35mm DSLRs cameras just happen to capture only a smaller portion of the image than 35mm film or a full-frame 35mm sensor would capture.

In my opinion, talk about Circle of Confusion, Circle of Confusion Diameter Limit, and the like contribute nothing to a beginning digital photographers understanding about how FOV, DOF, and subject magnification are affected by the APS-C size sensor in a Canon D30/D60/10D/20D/etc. relative to a full-frame sensor.

Camera sales people often tell them that their telephoto lenses get a 1.6x multiplier effect. They tell them that their 300mm lens becomes the equivalent of a 480mm lens. They tell them you get more DOF with the smaller sensor. Etc. At best, the camera sale people misunderstand. At worse, they downright defraud people.

You do not need to know anything about CoC or COCDL to understand that the only thing that changes is FOV when the sensor is smaller.

What you do about that smaller FOV is a separate issue.

How you calculate DOF and the role of CoC is also a separate issue.

The only difference between a larger sensor and a smaller sensor -- everything else being constant (i.e., the same distance to the subject, the same aperture, the same lens, the same focal length) -- is just one thing: FOV.

You can say, most people will reframe. I have no idea what "most" people will do. But when they do reframe -- large sensor or small sensor -- things are gonna change. That's not because of sensor size. That's because you reframed: something completely separate and independent of sensor size.

Cheers,

Mitch
Title: Sensor size and DoF
Post by: gkramer on July 08, 2006, 01:45:16 PM
Sensor size doesn't affect DoF; the Nikon D2X in effect has two sensor sizes, the 12MP APS-C sensor, and in "High-speed crop mode", a smaller 6MP sensor obtained by masking out about half the sensor area. The resulting image, in terms of DoF and everything else, is completely equivalent to doing an identical crop in Photoshop of the full-sensor image. (Nikon's purpose in doing the crop in-camera is to reduce file size to increase the fps rate.) The same is true in comparing a full-frame sensor with an APS-sized one.
Title: Sensor size and DoF
Post by: BJL on July 09, 2006, 05:59:10 AM
Quote
I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc.  But I don't know why.  Could I persuade anyone to enlighten me, please?
[a href=\"index.php?act=findpost&pid=65745\"][{POST_SNAPBACK}][/a]
This has been discussed many times in this forum, and I believe there is a consensus that:

1) If you take photos from the same distance and compare DOF on equal sized prints viewed from an equal distance, the DOF will be the same for any combination of focal length f and f-stop N that gives the same effective aperture diameter, A=f/N. For example, f=50mm, f/1.4, f=70mm f/2 and f=100mm f/2.8 all have effective aperture diameter about A=35mm, and so all give about equal DOF in a "same print size, same distance" comparison.

In other words, adjust f-stop in proportion to focal length to get equal DOF.

2) Smaller formats typically involve using shorter focal lengths to get the same framing of the subject (same FOV), and so will get the same DOF with a proportionately lower f-stop (and thus potentially with a lower ISO speed), or will give more DOF with the same f-stop.

Sometimes the "shorter, brighter" lens needed for equally low DOF does not exist for a smaller format (50/1.4, 85/1.2 or 85/1.4 in 35mm have no matches for for smaller formats). Other times a "match" does exist and then is usually similar in cost and weight, due to having similar sized lens elements, due to the similar aperture diameter (135/2 vs 85/1.4, 200/2.8 vs 135/2, 300/4 vs 200/2.8, 300/2.8 vs 200/2, 600/4 vs 400/2.8).


Historically, lenses for larger formats have typically had longer focal lengths and higher minimum f-stops, and have been most often used with higher f-stops in order to meet DOF needs, leading to the pattern of larger formats typically being used with lower shutter speeds, or alternatively higher ISO speeds to get the same DOF and shutter speed.
Title: Sensor size and DoF
Post by: Ray on July 09, 2006, 06:22:38 AM
Quote
Historically, lenses for larger formats have typically had longer focal lengths and higher minimum f-stops, and have been most often used with higher f-stops in order to meet DOF needs, leading to the pattern of larger formats typically being used with lower shutter speeds, or alternatively higher ISO speeds to get the same DOF and shutter speed.
[a href=\"index.php?act=findpost&pid=70137\"][{POST_SNAPBACK}][/a]

What I say is, 'roll on the new technology currently under development to circumvent the laws of diffraction'.  
Title: Sensor size and DoF
Post by: AJSJones on September 03, 2006, 03:44:13 PM
Quote
....

Imagine that you took 35mm film and painted an opaque back ink frame around the film so that only the central 2/3 of the film frame is exposed to the light during exposure. Do you believe you get subject magnification or affect DOF? Well, you wouldn't.  Ask anyone who's had the film transport in a 35mm film camera go screwy.

A less than full-size sensor has no sensors around the periphery of a 35mm frame, just as if you blocked the extra sensors of a full-frame sensor with something opaque. That's the only difference in a 35mm DSLR. All you get is reduced FOV. You do not get a boost in magnification, subject or otherwise.   

Cheers,

Mitch
[a href=\"index.php?act=findpost&pid=66803\"][{POST_SNAPBACK}][/a]

Mitch,
To continue with your scenario and keep the "comparison" valid (i.e. not introduce new variables), you have to switch painted and unpainted frames in the same enlarger (and not change anything else in it) to make the prints.  Guess what, the cropped frame print is smaller.  Yup, same as cutting down the original print to leave the central 2/3.    It was the same image coming through the lens, so this is rational.

Now, from the same viewing distance, you are correct that each will have the same parts appear to be in focus and nothing will be different about the central 2/3 of the image.  So the "All you get is reduced FoV" comment must apply to the print as well. DoF as a physical thing doesn't exist until you view the image because HOW you view the image determines what will appear in focus aka acceptably sharp.

 "A beginning digital photographer" , perhaps someone coming from FF 35mm such as film, to cropped sensor such as digital needs to appreciate this consequence, don't you think?  Should there be a warning like "Attention, this camera makes smaller prints than you are used to"    

Andy
Title: Sensor size and DoF
Post by: Ray on September 03, 2006, 08:29:21 PM
It's odd that there still seems to be confusion over the effects of a cropped sensor after all this time. It's probably due to the unfortunate terminology of 'crop factor' which has resulted from the fact that APS-C format cameras all use 35mm lenses.

If you could fit Medium Format lenses to 35mm bodies, then we could call the 35mm format a 'cropped MF format' and we'd all be confused about which lens to use.  
Title: Sensor size and DoF
Post by: Chris_T on September 04, 2006, 08:19:13 AM
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What do I have to lose?  I'll jump in to the fray  

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF.  So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

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I'm no DOF expert and don't even own a digital camera. So I'm qualified to jump in as well. <g>

Your statement makes most common sense to me, and is what I would expect from digital cameras with small or full frame sensors.

Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?
Title: Sensor size and DoF
Post by: Ray on September 04, 2006, 10:36:28 AM
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Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?
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Your question is confused. In the first sentence you write, 'let's leave small and full frame sensors out'. In the second sentence you bring 'full frame sensors' back in. Which is it? Out or in?
Title: Sensor size and DoF
Post by: Chris_T on September 05, 2006, 09:39:30 AM
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Your question is confused. In the first sentence you write, 'let's leave small and full frame sensors out'. In the second sentence you bring 'full frame sensors' back in. Which is it? Out or in?
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I meant instead of comparing DOF differences between small and full frame sensors, will there be any DOF differences for a lens when used on a 35mm film camera and when used on a 35mm digital camera with full frame sensor? That's a mouthful, hope I got it right this time.
Title: Sensor size and DoF
Post by: Ray on September 05, 2006, 10:04:56 AM
Quote
I meant instead of comparing DOF differences between small and full frame sensors, will there be any DOF differences for a lens when used on a 35mm film camera and when used on a 35mm digital camera with full frame sensor? That's a mouthful, hope I got it right this time.
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Only in respect of differences in resolution at the plane of focus. It's theoretically possible that a fine grain B&W film could deliver more resolution than, say a 5D, but color film wouldn't stand a chance.

DoF calculators do not bring such matters into their calculations, but it seems clear to me that either a lens or film that is able to deliver greater resolution at the plane of focus than a lesser lens or lower grade film (or coarser sensor), will produce an effect of less DoF because the difference between what's in focus and what's out of focus will be greater.
Title: Sensor size and DoF
Post by: Jonathan Wienke on September 05, 2006, 01:23:59 PM
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Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?

The correct answer is a definite "maybe". Because of it's higher resolving power, the digital sensor has a smaller effective circle of confusion, and therefore less DOF, but only if the lens used is as good as the sensor. If your lens is a cheap coke bottle that is outresolved by the film and digital sensor, DOF will be identical between digital and film. But if you have a good lens that outresolves the film but not the digital sensor, then the more sharply-focused areas found only in the digital image will raise the bar of what appears to be "in focus", and areas that appear "in focus" in the film image will suffer in comparison to other areas in the digital image that are even more sharply focused, and therefore will appear "out of focus".

What we perceive as "in focus" is based on comparison to other parts of the image, so any area that is very sharply focused will cause less sharply-focused areas to appear "out of focus". So the more resolution you extract from a given imaging area (like 24x36mm), the narrower your DOF will be--you're effectively decreasing your CoC size when you do so. As prints get larger, the difference will become more noticeable. Here's an example:

(http://visual-vacations.com/images/2006-04-29_0042.jpg)

In the web-sized JPEG above, everything appears to be "in focus" because the entire image is equally sharp--resolution is limited by downsampling to 800x640 pixels, not the lens or sensor. But when looking at the full-resolution version of this image, you can see that the right edge of the image is slightly OOF compared to the center and left side. By reducing overall resolution when downsizing this image for web display, I increased the effective CoC and increased DoF. But when increasing resolution, (going back to the original sized image) I decrease the effective CoC and DoF decreases proportionately.
Title: Sensor size and DoF
Post by: Chris_T on September 06, 2006, 08:35:32 AM
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The correct answer is a definite "maybe". Because of it's higher resolving power, the digital sensor has a smaller effective circle of confusion, and therefore less DOF, but only if the lens used is as good as the sensor. If your lens is a cheap coke bottle that is outresolved by the film and digital sensor, DOF will be identical between digital and film. But if you have a good lens that outresolves the film but not the digital sensor, then the more sharply-focused areas found only in the digital image will raise the bar of what appears to be "in focus", and areas that appear "in focus" in the film image will suffer in comparison to other areas in the digital image that are even more sharply focused, and therefore will appear "out of focus".
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Now it's my turn to be confused. When "less DOF" is used, I always take it to mean that the distance range within which everything is sharp is *shorter*. But in your explanation above, you seem to say that a digital camera will have a *longer* distant range within which everything is sharp than a film camera, everything else being equal.
Title: Sensor size and DoF
Post by: Jonathan Wienke on September 06, 2006, 11:40:15 AM
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But in your explanation above, you seem to say that a digital camera will have a *longer* distant range within which everything is sharp than a film camera, everything else being equal.

Re-read what I wrote: Digital achieves higher resolution, smaller CoC, narrower DOF. The sharper the sharpest focus is, the narrower the distance range that will be that sharply focused in the image.
Title: Sensor size and DoF
Post by: Ray on September 06, 2006, 07:15:33 PM
In broad terms, if the lens and f/stop is the same and the distance to subject is the same then the DoF will be the same, whether film or sensor. However, if you wish to put a fine point on it, there will be subtle differences in DoF depending upon the resolving power of the film or sensor being used. For example, a 1Ds2 should produce a marginally shallower DoF than the 5D, but so marginally shallower that you probably wouldn't notice it.

Likewise, a T-Max ISO 100 B&W film, that can apparently resolve 100 lp/mm at 60% MTF would produce a shallower DoF (everything else being the same) than an average slide or color film, given a big enough print.
Title: Sensor size and DoF
Post by: George Barr on September 08, 2006, 09:09:07 AM
Depth of field is simple. The mathematical formula for final sharpness in the print has depth of field inversely proportional to magnification from sensor to print (you make bigger prints, you have less depth of field, you use smaller sensor to make same size print, you have less depth of field), BUT, depth of field is inversely proportional to the SQUARE of the focal length, thus changes in the focal length of the lens used (the actual focal length, not some fudged 35 mm. equivalrent) have a much larger effect on depth of field.

This applies no matter what sensor size you have, whether it's film or digital, contact print or enlargement, sharpness fanatic or prebyopic baby boomer who's lost his reading glasses.
Title: Sensor size and DoF
Post by: Ray on September 08, 2006, 11:33:32 AM
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Depth of field is simple. [a href=\"index.php?act=findpost&pid=75858\"][{POST_SNAPBACK}][/a]

DoF is not simple. It's complicated by all sorts of factors which have an influence one way or another, including print size, lens quality, focal length, sensor quality, sensor size, objective factors and subjective factors.

There have been many protracted debates on this forum about these issues, some arguments addressing only the properties of a perfect lens using simplistic mathematical formulas and other arguments taking in a broader spectrum of variables that have both a direct and indirect influence on DoF.

Some folks will argue that sensor size has no bearing on DoF, for example, completely ignoring the practical reality that sensor size determines choice of focal length and therefore indirectly affects DoF. But because sensor or film format does not appear in the DoF formulas, such people will strenuously deny that sensor size has any bearing on DoF. It's a sort of photographic fundamentalism.
Title: Sensor size and DoF
Post by: Olivier_G on September 09, 2006, 02:13:19 PM
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DoF is not simple. It's complicated by all sorts of factors which have an influence one way or another, including print size, lens quality, focal length, sensor quality, sensor size, objective factors and subjective factors.
Most of the debates I see about "DOF" could be sum up as "how do you define DOF?". Many people use different meanings... without even being aware of this...  

I like the traditional way (http://www.dofmaster.com/) (ie: DOF formulae and CoC=1/1730 or whatever of the diagonal format) because its mainstream since years and provides also a good comparison basis for Background blur, even between different formats.
In another forum, I had to define "useful DOF" because people wanted to take into account the system real limitations and even "absolute DOF" to please one who considered DOF to be a characteristic of the lenses only... Oh well...  

Olivier
Title: Sensor size and DoF
Post by: Ray on September 09, 2006, 08:16:55 PM
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(ie: DOF formulae and CoC=1/1730 or whatever of the diagonal format)


Olivier,
I take it you mean CoC should be 1/1730 of the diagonal of the format. I suppose that's good enough for an 8x12" print, although some demanding people will claim the CoC should be equal to the pixel pitch of the sensor if maximum sharpness at the plane of focus is desired on the maximum size print the camera can produce, without interpolation, at 240 ppi.

Where I find these DoF calculators begin to break down is in the area of lens quality and sensor pixel pitch. For example, DoF Master treats the D30 the same as the 30D. The sensors are the same size. However, the D30 has 3mp and the 30D 8mp, yet DoF Master uses the same CoC of 0.019mm to get the same result at the same aperture and focal length of lens and same distance to subject.

I suppose this is fair enough if you make an assumption that the D30 with an average lens will deliver detail as sharp as the eye can see when viewing an 8x12" print from a distance of 10" or so, and that any sharper result the 30D might produce with a better lens on a larger print is irrelevant for this size of 8x12" print.

Perhaps someone who owns both a D30 and 30D could test this   .

I notice that DoF master does not emphasise that their figures apply only to a print size of 8x10" (or 12"). In fact I can't see it mentioned. But maybe I haven't looked hard enough.
Title: Sensor size and DoF
Post by: AJSJones on September 09, 2006, 11:12:24 PM
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Perhaps someone who owns both a D30 and 30D could test this   .

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Ray, I started with a D30 and have a 20D now.  Without formally doing any comparisons , my recollection/experience tells me the following.  I would say that 8x12's from the D30 with a good (enough) lens - I had a good 28-135 - came close to that performance.  I also had some poor lenses that didn't - suggesting something about system resolution and that it didn't take much loss in lens resolution for the overall loss to be noticeable;  i.e. the D30 was at its limit at 8x12 at 240ppi original pixels, my (preferred) limit.  The 20D goes beyond 8x12 at 240.  

The DoF concept is definitely a moving target, given the variability in print size, degree of crop/enlargement that's so easy with digital.  With the temptation to use original pixels at the printer's limit making it so easy to either crop/enlarge or just print bigger, it makes sense to use the pixel (or close to it) as the CoC to guide capture apertures during exposure

Andy
Title: Sensor size and DoF
Post by: Ray on September 10, 2006, 01:16:20 AM
Andy,
As you describe it is pretty much as I imagined, but a thorough camparison of the D30 and 30D on an A4 size print would be interesting, especially if the D30 shot was taken with a zom at, say 50mm, and the 30D shot was taken with the Canon 50/1.4 prime.
Title: Sensor size and DoF
Post by: Olivier_G on September 10, 2006, 05:30:56 AM
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Where I find these DoF calculators begin to break down is in the area of lens quality and sensor pixel pitch.
Ray, it's just that "DOF" is defined without those.
Historically, DOF has been defined by using the 'perfect lens' formulae and a 1/x ratio of diagonal format that matched both an "acceptable" quality for viewing print AND the resolution of film used at that time.
Resolution of lenses and films and then digital cameras has improved, but this hasn't changed DOF definition... and for good reasons: simplicity, relationship too Out Of Focus Blur and probably the most important one => photos are meant to be viewed, not to be scrutinized with a microscope at 100%...  
That's why we keep finding it in most litterature and in all online DOF calculators. This is also what is used by all manufacturers (Canon, Schneider, Leica, etc...).
Using a different 1/x ratio of the format depending on one's needs is still within this definition.

Taking into account the Lens resolution, Sensor resolution, Diffraction, more accurate formulae depending on lens design, non planeity of focus area, whatelse... is a different modelization.
Anyone can use and refine such modelization if they want.
The only thing I am asking is to use a different name than "DOF/Depth Of Field" which is assumed to be the traditional definition (and is really mainstream) in order to avoid misunderstandings. This is why I suggested "Useful DOF", etc... for those different concepts.

Olivier
Title: Sensor size and DoF
Post by: Olivier_G on September 10, 2006, 05:57:02 AM
If you are interested in more comparisons, have a look at those shots (http://www.pictchallenge-archives.net/TESTNUM/bokeh1.html) by Jean-Marie Sepulchre and Michel Denis-Huot (in french). They are quite interesting about 24x36 vs APS, although Jean-Marie and I didn't agree on the conclusions...  

If you want to go down the 'Lens&Sensor quality' road, don't forget:
- you won't have any formulae to calculate that "useful DOF" (change name here...)
- using an excellent lens and a bad one won't give you the same "useful DOF" even when using exactly the same settings
- of course, "useful DOF" will be different in the center and in the corners where lenses are less sharp
- there won't be any relationship between DOF and OOF blur
- Diffraction will have to be taken into account (and will you get more DOF with larger CoC or something else like DOF=0... you choose)
- it will be sensor+lens dependant... and will have to be updated with every change
- don't count me in!!!  

Olivier
PS: if you do so, please don't forget to use a different name than just "DOF/Depth Of Field" so that I can understand what you're talking about and leave you alone in that mess...  
Title: Sensor size and DoF
Post by: Ray on September 10, 2006, 09:00:33 AM
Quote
If you want to go down the 'Lens&Sensor quality' road, don't forget:
- you won't have any formulae to calculate that "useful DOF" (change name here...)
- using an excellent lens and a bad one won't give you the same "useful DOF" even when using exactly the same settings
- of course, "useful DOF" will be different in the center and in the corners where lenses are less sharp
- there won't be any relationship between DOF and OOF blur
- Diffraction will have to be taken into account (and will you get more DOF with larger CoC or something else like DOF=0... you choose)
- it will be sensor+lens dependant... and will have to be updated with every change
- don't count me in!!!   


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Olivier,
I assure you I have no intention of going down that road. The DoF issue for me is mainly a conceptual one of academic interest and a matter of semantics and meaningful definition. As a practical photographer who uses zoom lenses most of the time, outdoors and in the field, I have no way, nor the time, to make precise DoF measurements which take into consideration lens resolution, pixel density or print size. Even using a basic DoF calculator such as DoF Master would be too cumbersome for me. For example, if the nearest point which is reasonably sharp is 12.8 metres, how do I measure that distance? I'm not interested in carrying around a 30 metre measuring tape or wading acroos a river to make a precise measurement.

It is sufficient for me simply to be aware of the factors that can influence the perception of DoF on the final print. For example, with my D60 and 20D I was more reluctant to use f16 than I am with my 5D because the greater pixel density of the 20D revealed the softening effects of f16 more. With the 5D the softening effect is hardly noticeable. F16 is very usable with the 5D because the pixel density is less. At the same time, I need to use f16 with the 5D more than I do with the 20D because DoF is shallower with the 5D at the same f stop and FoV.

These are issues I need to be aware of to make reasonably accurate guesses in the field.
Title: Sensor size and DoF
Post by: danag42 on September 25, 2006, 10:24:14 PM
Perhaps instead of using the "focal length multiplier factor" we should simply call it "crop factor".  For that's what it is.

Take your 4x5 camera, put a 200mm lens on it.  Then cut out a 24x36mm piece.  The result is EXACTLY THE SAME as if you used the same film in a 35mm camera with a 200mm lens at the same f/stop, from the same viewpoint.

It's a good thing that our large format lenses go up to f/64, f/96, and further.  That allows us to get more depth of field from the same angle of view.

So an APS-C sensor is the same as if you cut out an APS-C size bit of film from a 35mm camera.  

The "normal" lens for 24x36 is usually 50mm (it's actually 43mm, but that's another whole subject).  The "normal" lens for APS-C is 31mm or thereabouts.  So to get the same field of view, from the same spot, you're using a wider angle lens on the smaller sensor.  Thus more depth of field, assuming the same f/sop.
Title: Sensor size and DoF
Post by: Craig Arnold on September 26, 2006, 05:25:30 PM
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Perhaps instead of using the "focal length multiplier factor" we should simply call it "crop factor".  For that's what it is.

Take your 4x5 camera, put a 200mm lens on it.  Then cut out a 24x36mm piece.  The result is EXACTLY THE SAME as if you used the same film in a 35mm camera with a 200mm lens at the same f/stop, from the same viewpoint.

It's a good thing that our large format lenses go up to f/64, f/96, and further.  That allows us to get more depth of field from the same angle of view.

So an APS-C sensor is the same as if you cut out an APS-C size bit of film from a 35mm camera. 

The "normal" lens for 24x36 is usually 50mm (it's actually 43mm, but that's another whole subject).  The "normal" lens for APS-C is 31mm or thereabouts.  So to get the same field of view, from the same spot, you're using a wider angle lens on the smaller sensor.  Thus more depth of field, assuming the same f/sop.
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Which is true, as far as it goes, and completely misses the point.

Why would someone do that? Do you do that? What people do is frame the shot they want through the viewfinder and then enlarge it to the print size they want. If they can't get the framing they want then, if at all possible, they either change lens or move their camera. What they don't want to do is crop a bit out of the middle of their picture and thereby lose a significant percentage of the available resolution on whatever format they happen to be using.

Focal length multiplier is a far more useful term, not because anyone cares very much about focal lengths, but because everyone is really far more interested in angle-of-view, and people know (in broad terms) what angle-of-view corresponds to a particular focal length on a 35mm camera.

So insisting that 200mm is 200mm is 200mm is pedantic and misleading and frankly has nothing to do with the way pictures are taken.

But if it makes you feel intellectually superior...

(The bit that is unspoken and wrong in what you wrote is that you're implicitly assuming that the print size varies between the crop and non-crop. If you make an 8x10 print from a 200mm lens on APS-C and an 8x10 print from a 200mm lens on 24x36mm sensor and crop out the equivalent bit, you are not left with an 8x10 print. If you enlarge that middle bit you have lost your "identical" DOF.)
Title: Sensor size and DoF
Post by: Anon E. Mouse on September 27, 2006, 01:00:03 AM
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Focal length multiplier is a far more useful term, not because anyone cares very much about focal lengths, but because everyone is really far more interested in angle-of-view, and people know (in broad terms) what angle-of-view corresponds to a particular focal length on a 35mm camera.

Actually, "focal length multiplier" is a bad term because focal length changes more than angle of view - "crop factor" is far better. This problem of different formats is only a problem with amateurs with a singular knowledge of 35mm cameras. Photographers used to other format do not have any isses with this. Manufacturers have taken this "35mm equivalent" thing because they have found their customers do not understand fairly basic concepts in photography. They have even gone so far as to give magnification in "35mm equivalents."

This thread is a result of the "dumbing down" of photography inforced by manufacturers and the trade publications. The issue here is not that complex, but the subject come up time and again. Actually, there should not be any reason to compare formats. Learn to use the format as it is and not compared it to something else.
Title: Sensor size and DoF
Post by: howiesmith on October 09, 2006, 04:50:41 PM
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I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc.  But I don't know why.  Could I persuade anyone to enlighten me, please?
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"There was a query ... as to whether Depth of Field was calculated any differently for digital Vs. film.  The answer is, no.  There is no difference whosesoever.  DOF doesn't care about the recording media type or size, though a lower COF is used for medium and large format, since the amount of magnification to make a decent sized print is much less than for 35mm."

I think that should answer the original question.

The last part of the last sentence in the quot refers to the degree of enlargement.  Medium and large formats usually require less enlargement.  Therefore, I think the quote should be that a higher COF is used, not a lower.

Oh, the quote is from Michael Reichmann on the Luminous-Landscape.
Title: Sensor size and DoF
Post by: davaglo on October 09, 2006, 09:59:13 PM
If one were to place on a multi-camera bar a 20D and a 5D using the same mm lens at the same f stop the picture would be exactly the same except for the crop factor of the aps sensor, right? At the same time, the DOF is only a function of the distance from the sensor to the subject at a given f stop, right? The shorter the distance from the sensor to the subject the shalower the DOF , the longer the distance the deeper the DOF. Have I got it?

Jerry
Title: Sensor size and DoF
Post by: Anon E. Mouse on October 09, 2006, 11:53:56 PM
Quote
If one were to place on a multi-camera bar a 20D and a 5D using the same mm lens at the same f stop the picture would be exactly the same except for the crop factor of the aps sensor, right? At the same time, the DOF is only a function of the distance from the sensor to the subject at a given f stop, right? The shorter the distance from the sensor to the subject the shalower the DOF , the longer the distance the deeper the DOF. Have I got it?

Jerry
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Almost. You are right that depth of field increases with object distance and larger f-numbers. However, if the images are displayed the same size, the depth of field will be sligthly less with the smaller sensor due to the greater image magnification.

Normally, photographer are only interested in one camera at a time so cross format comparisons are not normally an issue. But note, if the focal length is different where the angle of view is the same, the smaller sensor will have the greater depth of field.
Title: Sensor size and DoF
Post by: Ray on October 10, 2006, 01:19:29 AM
Quote
If one were to place on a multi-camera bar a 20D and a 5D using the same mm lens at the same f stop the picture would be exactly the same except for the crop factor of the aps sensor, right? At the same time, the DOF is only a function of the distance from the sensor to the subject at a given f stop, right? The shorter the distance from the sensor to the subject the shalower the DOF , the longer the distance the deeper the DOF. Have I got it?

Jerry
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This is an excellent example of the sort of statement that brings together subjective and objective concepts of DoF which are the source of much confusion.

Quote
....the picture would be exactly the same except for the crop factor of the aps sensor, right?

That's almost like saying, 'these two pictures are exactly the same apart from the fact they are different'.

If you compare a 5D image with a 20D image in the manner you suggest, what is the impression you get if the 5D image, with its wider FoV, has a few out-of-focus rocks in the foreground which have been cropped out of the 20D image?

Subjectively, whether 2 images are completely different or similar, the image containing blurred objects in the foreground, background or to one side, will be perceived as having shallower DoF. If you crop the 5D image so the FoV is exactly the same as that of the 20D, you are comparing a 5mp image with an 8mp image. You don't have to be a genius to work out which image is likely to be sharper.

To the extent that the 20D image is sharper than the 5D image at the plane of focus, the 20D image will have a shallower DoF.

Your above statement would be more true if the comparison was between a cropped 1Ds2 image and a D60 image, same lens, same f stop, same distance to subject, same pixel density.

A couple of days ago I used my TS-E 24mm with the 5D to get an extended panoramic shot of one of these magnificant ruins in the jungle at Siem Reap (Preah Khan, actually), by extending the width 22mm using shift. Back at the hotel, having downloaded my day's shooting to the laptop, I was dismayed to find the corners and edges were not sharp. I've been used to using this lens with my D60 and 20D where there's no problem with resolution fall-off at the edges.

It's really not acceptable. I wish Canon would upgrade this lens. It's expensive and we deserve better. I felt compelled to return to the same venue this morning and reshoot using my 20D with the same lens but from a greater distance.
Title: Sensor size and DoF
Post by: howiesmith on October 10, 2006, 05:53:45 AM
Ray makes a good point.  DoF is a visual thing.  But it can calculated.  He also brings up some things that are second order effects.

When breaking down something with several variables to see the effect of s single variable or each variable, it is very useful to seperate the variables and change only one at a time.  I am afraid Ray and others are guilty of confusing the issue by changing more than one variable at a time.  If you want to see the effect of format (sensor size), change just the sensor size.  Looking at different size prints (showing the out of focus rocks) and making prints of different subject image magnification same size prints from different size sensors) only confuses the answer and hides the effect of sensor size among other factors.

According to Michael Reichmann, changing only sensor size does not change depth of field.  I believe that is true.

The original question as posed changes more than one varible (sensor size and sensor resolution by using two different kinds of cameras).  If the original question is what effect does sensor size have on depth of field, the answer is none whatsoever.

As an aside, pinhole cameras may have huge depth of field for at least two reasons.  The f/stop is huge (tiny aperture with long focal length).  The resolution is poor so it is difficult for people to distinguish betweem focused and out of focus parts of the image (the very essence of depth of field).  Depth of field is judged by what the viewer sees as acceptable sharp.  Some may say a pinhole camera (or even a Canon 1ds with a cheapo coke bottle lens) has no acceptably sharp image, even at the plane of focus.  Then the depth of field is undefined (for that viewer).
Title: Sensor size and DoF
Post by: Ray on October 10, 2006, 06:26:32 AM
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According to Michael Reichmann, changing only sensor size does not change depth of field.  I believe that is true.
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I'm familiar with this view. There is a theoretical DoF in relation to a perfect or standard lens that can be calculated with a simple mathematical formula. The result is the DoF of the imaginary lens, not the printed image.

It's ironic that we should spend  so much time worrying about the performance of our lenses, yet when it comes to DoF, many of us (not me, however), accept that all lenses are the same.

Changing only sensor size does not change the characteristics of the lens, if the lens is not changed, but it may certainly change the charcteristics of the sensor (pixel pitch, for example) and it certainly, without doubt, changes the composition of the scene being photographed.

When Michael made that statement, in my opinion he should have emphasised that he was only referring to the characteristics of the lens in an objective manner. Clearly, changing sensor size cannot affect the properties of the lens. Such properties are independent of the photograph or composition.
Title: Sensor size and DoF
Post by: Anon E. Mouse on October 10, 2006, 10:39:28 PM
DOF is calculated based on standard print size not based on the image at the image plane of the camera (you can do it, but it is still in relation to display size). This is done because DOF is based on the average angular resolution of the human visual system - there is no absolute definition of sharpness. To say DOF is exactly the same at the image plane regardless of the format size is a pointless comment; are you expecting the resulting DOF in my full-frame print from a 55mm Grandagon on my 4x5 view camera to appear the same from my 55mm macro Nikkor on my F3? Or do you always view images at the sensor dimensions - I would suggest you are very, very near sighted as well as that limit being really tough for folks with compact digital cameras.
Title: Sensor size and DoF
Post by: howiesmith on October 11, 2006, 12:32:01 AM
Quote
DOF is calculated based on standard print size not based on the image at the image plane of the camera (you can do it, but it is still in relation to display size). This is done because DOF is based on the average angular resolution of the human visual system -
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It is not necessary to base DoF on an average angular resolution of the human visual system.  It is merely a convenience.  Because DoF is personal, I can base it on anything I want, like my own visual system.  I might have the eyes of an eagle or Mr. Magoo.  Doesn't matter.  If I do use the average person, my prints will appeal to the average person, even if that persom does not exist or view my prints.

Same goes for a standard print size.  I have no need to limit myself by that assumption.  The driving factor is the degree of enlargement for a particular negative or file and viewing conditions.  I definiely do not need to assume I am making an 8x10 or 5x7 print to be viewed by the average person at arms length.  As mentioned in Michael Reichmann's tutorial, various lens makers use various assumption.

When I make prints for myself, I use what works for me.  I have less than average eyes.  For sales, I use the average.  Don't know who, if anyone, is going to buy or under what conditions they will view the print.  Sales are a crap shoot.
Title: Sensor size and DoF
Post by: Ray on October 11, 2006, 09:14:49 AM
Quote
It is not necessary to base DoF on an average angular resolution of the human visual system.  It is merely a convenience.  Because DoF is personal, I can base it on anything I want, like my own visual system. 
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This is the crux of the problem. Objective factors can be more easily defined. We can say that a lens at a particular f stop and distance to subject will produce images that are acceptably sharp between distances of x and y from the camera.

Acceptably sharp to whom? Acceptably sharp at what print size? Acceptably sharp at what viewing distance from the print? The variability of the combination of these 3 factors is enormous. We try to simplify matters by introducing a standard size print about the size of a page from a largish book (8x10"); make an assumption that everyone's eyesight is roughly the same for reading purposes because if it's not, we'd expect it to be corrected by appropriate glasses.

The more precise you want to be about this issue, the more variables you have to take on board.
Title: Sensor size and DoF
Post by: howiesmith on October 11, 2006, 09:53:24 AM
Quote
Acceptably sharp to whom? Acceptably sharp at what print size? Acceptably sharp at what viewing distance from the print? [a href=\"index.php?act=findpost&pid=79929\"][{POST_SNAPBACK}][/a]

Whom?  The photographer or print designer.
Print size?  The one chosen by the photographer or print designer.
Viewing distance?  The one chosen by the photographer or print designer.

Then throw in "viewer."  The one chosen by the photographer or print designer.

DoF is simple.  The photographer or print designer (usually rhe same person) just has to know, understand and plan (apply the principals) what he is doing.  A print may look one way to me but totally different to you or anyone else.  It may have more or less DoF than I planned for you or anyone else.  I don't think I ever said this was a recipe for a print everyone would like.  Just me.

DoF is no different than any other photographer selected print element.  There are no guarentees about subject, exposure, lighting, or composition either.
Title: Sensor size and DoF
Post by: Olivier_G on October 11, 2006, 11:46:08 AM
Some comments:
Quote
According to Michael Reichmann, changing only sensor size does not change depth of field.  I believe that is true.
Michael Reichmann actually said (http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml) that DoF calculations for Digital were identic to Film... which means that a smaller format has a smaller CoC (and DoF will be different).
The quote, again:
Quote
whether Depth of Field was calculated any differently for digital Vs. film. The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, though a lower COF is used for medium and large format, since the amount of magnification to make a decent sized print is much less than for 35mm.

Ray,
Quote
If you crop the 5D image so the FoV is exactly the same as that of the 20D, you are comparing a 5mp image with an 8mp image.
To the extent that the 20D image is sharper than the 5D image at the plane of focus, the 20D image will have a shallower DoF.
Quote
There is a theoretical DoF in relation to a perfect or standard lens that can be calculated with a simple mathematical formula.
Changing only sensor size does not change the characteristics of the lens, if the lens is not changed, but it may certainly change the charcteristics of the sensor (pixel pitch, for example) and it certainly, without doubt, changes the composition of the scene being photographed.
Quote
The more precise you want to be about this issue, the more variables you have to take on board.
You are getting into the situation I described earlier (http://luminous-landscape.com/forum/index.php?showtopic=10934&view=findpost&p=75971).
And as I said, please use a different name to that concept, wich is not DoF™ (as defined and currently used by 99% of photographers and manufacturers). It will help avoid collisions and misunderstandings.

Olivier
PS: Angkor is a wonderful area (I stayed 7 days on site), especially for sunrise near temples you already visited by day.
Title: Sensor size and DoF
Post by: Olivier_G on October 11, 2006, 12:00:23 PM
Side-note: this article (http://www.bobatkins.com/photography/technical/bokeh.html) about Blur quality (Bokeh) and Blur quantity is extremely interesting, and may be of interest even for DOF fanatics here...  

Olivier
Title: Sensor size and DoF
Post by: howiesmith on October 11, 2006, 12:10:00 PM
Quote
Some comments:
Michael Reichmann actually said (http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml) that DoF calculations for Digital were identic to Film... which means that a smaller format has a smaller CoC (and DoF will be different).
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The value given to CoC is chosen by the photographer or print designer tp meet the needs of the designed print.  

It is not attached to any camera format.  Smaller formats (than what?) can have any CoC the print designer desires.  Small formats may require more enlargement to get the designed print size and therefore reuire a small CoC.  But a large format may also require a small CoC for a very large print to be viewed closely.  Likewise, a large CoC can be used with smallr format digital cameras to get shallow DoF.

CoC only depends on what appears to be in focus to the designer of a particular print under particular conditions for a particular viewr.  Or what the designer may think appears in focus.  When a print designer doesn't understand his audience and/or tries to satisfy a large audience with different needs, he may resort to assumptions (CoC, print size, viewing distance, eye sight, etc.) that do not apply to everyone (or anyone) in that audience.

My opinion is, simply stated, neither DoF calculations nor application has any connection to camera format.
Title: Sensor size and DoF
Post by: Anon E. Mouse on October 11, 2006, 11:35:58 PM
Quote
The value given to CoC is chosen by the photographer or print designer tp meet the needs of the designed print. 

It is not attached to any camera format.  Smaller formats (than what?) can have any CoC the print designer desires.  Small formats may require more enlargement to get the designed print size and therefore reuire a small CoC.  But a large format may also require a small CoC for a very large print to be viewed closely.  Likewise, a large CoC can be used with smallr format digital cameras to get shallow DoF.

CoC only depends on what appears to be in focus to the designer of a particular print under particular conditions for a particular viewr.  Or what the designer may think appears in focus.  When a print designer doesn't understand his audience and/or tries to satisfy a large audience with different needs, he may resort to assumptions (CoC, print size, viewing distance, eye sight, etc.) that do not apply to everyone (or anyone) in that audience.

My opinion is, simply stated, neither DoF calculations nor application has any connection to camera format.
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Please give me a few examples how you calculate that based on format, print size, and viewing distance. I am interested in your system. I would also like to know how you deal with diffraction.
Title: Sensor size and DoF
Post by: Ray on October 12, 2006, 03:47:46 AM
Quote
The value given to CoC is chosen by the photographer or print designer tp meet the needs of the designed print. 

It is not attached to any camera format. [a href=\"index.php?act=findpost&pid=79955\"][{POST_SNAPBACK}][/a]

This I think is your blind spot, Howie, or maybe it's my blind spot. It's true that the photographer, understanding the issues, can sensibly choose an appropriate CoC for a particular print size, to be viewed from a certain distance, enlarged from a specific sensor of given size and pixel density, using a lens at a particular f stop and focal length.

If the camera format is not attached to DoF considerations, then it becomes a bit tricky working out the degree of enlargement, doesn't it? I think it's possible with sufficient measuring devices and given all the other relevant information, including pixel count which, in digital systems, seems to take the place of enlargement.

For example, let's say I'm moving down from large format and am not at all familiar with miniature cameras. Someone plays a trick on me and hands me a Nikon D2X renamed as a Canon 5D with some Nikkor lenses also renamed as Canon lenses of the same focal length. I know the camera is a 12mp DSLR. Does it not make any difference what the sensor size is when I choose f stops for a particular DoF effect?
Title: Sensor size and DoF
Post by: howiesmith on October 12, 2006, 11:45:06 AM
Quote
For example, let's say I'm moving down from large format and am not at all familiar with miniature cameras. Someone plays a trick on me and hands me a Nikon D2X renamed as a Canon 5D with some Nikkor lenses also renamed as Canon lenses of the same focal length. I know the camera is a 12mp DSLR. Does it not make any difference what the sensor size is when I choose f stops for a particular DoF effect?
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Then all I can say is you don't know your tools.  How can you expect to build a house with a ballpeen hammer and a hacksaw?  It won't take you long to figure out something is really wrong.  If you don't understand your tools, it isn't very likely you will be able to use them to get what you want.

How can you expect to design prints if you don't understand your camera and lenses and how to use them?  My method does not require me to know the format but I do need to know the relationship between the viewed image size in the camera to the captured image size.  For both my cameras that is 1:1.  Yours may be different.  Format size is useful if you plan to make omly full frame enlargements.  Then just ratio up the format dimension to the print dimension.   I usually crop before enlarging.  You do need some way to measure the size of the image (the projected size) in the camera.  Otherwse, there is lots of math and measuring actual dimensions of things in the fireld - sometime not easily done.

I use 4x5 and 6x6 film and can measure image sizes directly from the ground glass.  No need to know the pixel size or pitch because there aren't any.  If there were, I don't need to know thatanyway.  (I do need to know not to try to make CoC less than the pixel size though.)  Then it is a simple matter of saying that the 1 inch high tree on the ground glass will be 20 inches on the planned print.  A 20X enlargement.  Not hard to do and not determined by format.  Note that if I am using my 4x5 and think I am using my 6x6, I still get the same results - a 20X enlargement.

Then I divide the CoC on the print by 20 to get the value to be used in planning the in camera work.  The rest is simple math using the easily determined factors like lens focal length, f/stop, focus distance, etc.  It is just a matter of taking time to plan what I'm doing.

I don't take a two week trip, squeeze off 10,000 images, and then print 50 "keepers."  I take a two week trip, maybe squeeze off 15 images, and get 1 or 2 I like.  Sometimes more, frequently less.  Sometimes none.  (But I enjoy the trip anyway.)
Title: Sensor size and DoF
Post by: howiesmith on October 12, 2006, 01:28:22 PM
Quote
Please give me a few examples how you calculate that based on format, print size, and viewing distance. I am interested in your system. I would also like to know how you deal with diffraction.
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Anon E. Mouse, I think from my reply tp Ray you can figure out "my system."  I use quotes because it isn't really mine.  This method is the result of several teachers, many books, soem personal experience and trial and error.

Viewing distance I frequently use the 1 minute of arch rule.  It usually works for me.  Mr. Magoo and a couple of my eagle friends don't agree always.  I may change it slightly for viewing conditions, like a dim room.

I deal with diffraction by simply ignoring it.  (I know it exists though and is quite a hurdle for some folk's styles.)  If I need f/22 1/2 (or 32 or 64) to get what I want, I use it.  I figure that is what its there for.  When I Schemphlug (never could spell that), rise/tilt/swing, I don't worry about using the side of the lens.  I just don't let that stand in the way.

Sometimes I want to rehoot an image.  Usually I can.  The light outside may change (for the better or worse).  If it changes for the better, I get a better image than I have, using the info I gathered that drove me to reshoot.  I have found trees don't grow very fast.

By the way, I never use depth of field preview on my SLR.  I have no idea what the "print" size or viewing distance is. and it is just too dim for me.  I only loupe 4x5 to check critical focus of the subject, not DoF.

DoF to me isn't something I can determine to a tenth of an inch.  I don't always get exactly what I want, as with most everything else.  If the result is unacceptable, I try to determine why and go on.  Reshoot if I can.

This method works for me and my style of shooting.  It probably isn't for everyone, maybe no one else.  But I do get anoyed when I'm told it doesn't work or nobody does that.  But then I don't photograph for anyone else but me.
Title: Sensor size and DoF
Post by: gkramer on October 12, 2006, 01:31:53 PM
Quote from: Ray,Oct 12 2006, 03:47 AM
"For example, let's say I'm moving down from large format and am not at all familiar with miniature cameras. Someone plays a trick on me and hands me a Nikon D2X renamed as a Canon 5D with some Nikkor lenses also renamed as Canon lenses of the same focal length. I know the camera is a 12mp DSLR. Does it not make any difference what the sensor size is when I choose f stops for a particular DoF effect?"

Yes, according to Wrotniak's M x A rule (http://www.wrotniak.net/photo/tech/dof.html), the DoF from an APC-sized sensor (with a magnification factor of M=1.6) at a given aperture A will be the same as that from a full-frame sensor at an aperture of M x A; in other words the full-frame Canon would have to use an aperture of f/12.8 to yield the same DoF as the D2X at f/8. This calculation takes account of the fact that the smaller sensor (or negative) will have to be enlarged 1.6 times more to yield the same final print size, and assumes that the images were taken with lenses of equivalent FoV (e.g., 100mmm for the D2X, 160mm for the full-frame Canon), and that the CoC (at the sensor) on which the DoF calculations are based is large relative to the pixel pitch (or film reslution), and compared to the Airey disk produced by lens diffraction (i.e. that we're shooting at a fairly large, non-diffraction limited aperture (f/8 is about the limit for the D2X, with its small pixel pitch).
Title: Sensor size and DoF
Post by: howiesmith on October 12, 2006, 02:25:30 PM
Quote
Does it not make any difference what the sensor size is when I choose f stops for a particular DoF effect?
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It makes no difference.

Suppose there is a person in the view finder showing head to foot.  I want a waist up print, so I will need to crop.  Now does it matter whether I throw away a half inch of negative or 2 inches of negative? (I use the word negative to denote either a piece of film or a computer file.)  I think not.  It only matters how much I am going to magnify (enlarge) the person to the final print.  If the 1" tall person on the negative is made to be 20" from waist up on the print, I think the enlargement will be about 40X, regardless of original format.

You can aslo run through the optical equations knowing only the peron is 6' tall and is Y feet away, I am using an Xmm lens and I want half of that person to be 20" tall on the print.  I won't bore you here with those details.

You say "Who would crop that much?  Change lenses or move up."  Then you have changed focal lenght of focus distance.  Anothr problem, but format is still eliminated the same way.

As an aside, in another part of this site there are some images for critique.  On at least one, there are suggestions to crop off about half the submitted image.  If the same size print is now made from half the original file, doesn't that change the DoF assumptions?  And then we still don't know (or care) what the origianal format was.  It may already be severely cropped or shown full frame or something in between.

There is a very lovely image submitted there by pom (some fall trees).  If he now decides to make a 48" high print of that same file, will the DoF be the same or different than that in the submitted image?  I know I view that imake on my computer monitor at abpout 18".  The computer image is back lit.  Does a print (front lit) 48" high hanging on my hallway wall in dim light have the same DoF when viewed from a maximum of 30" away?  I haven't run the numbers (or better yet, viewed the print) but I seriously doubt it.
Title: Sensor size and DoF
Post by: Ray on October 12, 2006, 08:37:37 PM
Quote
How can you expect to design prints if you don't understand your camera and lenses and how to use them?

I don't expect to be able to do that, but the issue we're discussing at the moment is the relevance of format to DoF calcualtions. If format is truly unattached to DoF considerations, then according to my simple understanding of logical processes, I don't need to know the format of my camera. I certainly don't need to know the format for compositions purposes using an SLR, because what I see through the viewfinder (or the EVF of a P&S) is what I get. If I have lots of experience using small cameras of various formats, I can make an educated guess as to the format of the camera. If I put a 50mm lens on a D2X with a 1.5x crop factor, which had been renamed as a FF Canon 5D, I might quickly sense that something was wrong and I'd probably be quite puzzled. (On the other hand, I probably wouldn't complain too much because all my prints would have a greater DoF than expected   ).

Quote
I use 4x5 and 6x6 film and can measure image sizes directly from the ground glass.  No need to know the pixel size or pitch because there aren't any.  If there were, I don't need to know thatanyway.  (I do need to know not to try to make CoC less than the pixel size though.)  Then it is a simple matter of saying that the 1 inch high tree on the ground glass will be 20 inches on the planned print.  A 20X enlargement.  Not hard to do and not determined by format.  Note that if I am using my 4x5 and think I am using my 6x6, I still get the same results - a 20X enlargement.

And would that 1" high tree on the ground glass be independent of format? What I'm trying to get from you, Howie, are the processes you would use to make a print of a particular size with a specified or designed DoF without having any knowledge of the format of the camera you used.

Quote
No need to know the pixel size or pitch because there aren't any.  If there were, I don't need to know thatanyway.  (I do need to know not to try to make CoC less than the pixel size though.)

Howie, in order to avoid making the CoC less than the pixel size, you need to know the pixel size or more precisely, pixel pitch.
Title: Sensor size and DoF
Post by: howiesmith on October 12, 2006, 09:15:40 PM
Quote
I don't expect to be able to do that, but the issue we're discussing at the moment is the relevance of format to DoF calcualtions. If format is truly unattached to DoF considerations, then according to my simple understanding of logical processes, I don't need to know the format of my camera. I certainly don't need to know the format for compositions purposes using an SLR, because what I see through the viewfinder (or the EVF of a P&S) is what I get. If I have lots of experience using small cameras of various formats, I can make an educated guess as to the format of the camera. If I put a 50mm lens on a D2X with a 1.5x crop factor, which had been renamed as a FF Canon 5D, I might quickly sense that something was wrong and I'd probably be quite puzzled. (On the other hand, I probably wouldn't complain too much because all my prints would have a greater DoF than expected   ).
And would that 1" high tree on the ground glass be independent of format? What I'm trying to get from you, Howie, are the processes you would use to make a print of a particular size with a specified or designed DoF without having any knowledge of the format of the camera you used.
Howie, in order to avoid making the CoC less than the pixel size, you need to know the pixel size or more precisely, pixel pitch.
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Ray, you don't need to know the format of the camera.  You only need to be able to measure the image size in the viewer and relate that to image size in the camera.  That is format independant.

The 1" high tree on the round glass is format independant, but does depend on the lens focal length and the distance from the tree to the camera.  It will be 1" high with my 6x6 with a 150mm lens and 1" high with my 4x5 and a 150mm lens.  But once on the ground glass, I don't care anyway.  The tree is an inch high on the ground glass and on the film, for my cameras.  It would only matter to me when selecting a lens and/or a camera-to-tree distance.

That's right Ray, I never use the camera's format or film size for designing prints.  Just the degree of enleagement I want from a focused image.  I could crop each negative with a pair scissors, get the same print without ever knowing how much negative I cut off and threw away.

If you use digital and happen to use a CoC smaller than the pixel pitch, the results will simplt not be as expected.  The same can be said of film and a CoC that cannot be recorded on the film.  It's a problem that may surface.  And frankly, since I don't use digital, I don't worry about it or a solution.  If I get a digital camera, I'll try to figure it out then.

Because I don't own a pixel, I need not know anything about its size.  You may if you try to make CoC too small for your camera to resolve.  I have not yet run into a case where my CoC was smaller than the lens/film resolution.  In fact, I don't even know what that is for my lenses and film selections.

Finally, greater DoF can be a problem if the print design calls for less.  Huge DoF isn't always the goal.  I am speaking of controlling DoF.  If all you want is maximum DoF. just use f/64 (of the biggest number you got) and tiny CoC (maybe pixel pitch) to calculate the hyprtfocal distance and work with the DoF equations to get the effect you want.  Oh, I forgot.  You fuss with diffraction.  Then use f/whatever no diffraction and hope for the best.
Title: Sensor size and DoF
Post by: Ray on October 12, 2006, 09:16:03 PM
Quote
You can aslo run through the optical equations knowing only the peron is 6' tall and is Y feet away, I am using an Xmm lens and I want half of that person to be 20" tall on the print.  I won't bore you here with those details.

Yes, I believe you can. If you have sufficient information about the subject being photographed and you have the means of measuring distances and angles and the means of measuring the size of objects in the viewfinder or on the ground glass of an LF camera, and you know the magnification relationship between the camera format and the image size in the viewfinder or ground glass, then you could also even dispense with knowledge of the focal length of the lens being used. You could work it out.

Could we then say that the focal length of the lens is unattached to DoF considerations?

Quote
As an aside, in another part of this site there are some images for critique.  On at least one, there are suggestions to crop off about half the submitted image.  If the same size print is now made from half the original file, doesn't that change the DoF assumptions?  And then we still don't know (or care) what the origianal format was.  It may already be severely cropped or shown full frame or something in between.

This is not relevant. All images without exception are cropped. Initially the image circle produced by the lens is cropped by the camera's format size and aspect ratio. The image might be further cropped in post processing, but that's not relevant to the discussion if it is assumed that the person doing the cropping knows the dimensions (format) of what's being cropped and knows the new dimensions (new format) of the resulting crop. I think that's a reasonable assumption, don't you?
Title: Sensor size and DoF
Post by: Ray on October 12, 2006, 09:31:30 PM
Quote
Ray, you don't need to know the format of the camera.  You only need to be able to measure the image size in the viewer and relate that to image size in the camera.  That is format independant.
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Isn't the image size in the camera the format?
Title: Sensor size and DoF
Post by: howiesmith on October 12, 2006, 09:47:25 PM
Quote
Isn't the image size in the camera the format?
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No.  Of course not.  Surely you know better.  Does the camera format change when I change lenses (or zoom) or move about?  I can increase the tree's size in the view finder by increasing the focal length (zoom in) or move closer.  All with the same camera and its fixed format.  I am not talking about the imaging size of the sensor or film.  I am talking about the focused image size of the subject.  It is focal length and distance dependant, but not camera format dependant.

If I use the same digital camera and look at the ficused image size in its view finder and tehn connect that same digital camera to a laptop computer, the tree size viewed will change but the camera format does not.  Now I need to know the relationship to the tree size on the computer screen to the tree size in the camera.
Title: Sensor size and DoF
Post by: gkramer on October 12, 2006, 10:05:51 PM
Quote from: howiesmith,Oct 12 2006, 09:15 PM
"Ray, you don't need to know the format of the camera... I never use the camera's format or film size for designing prints.  Just the degree of enleagement I want from a focused image.  I could crop each negative with a pair scissors, get the same print without ever knowing how much negative I cut off and threw away..."

If we're comparing two images shot with the same lens at the same aperture, same subject-to camera distance, etc., then sensor size (or film size) doesn't matter, since the smaller (APC-sized) sensor amounts to a crop of the larger one, and DoF etc. within the cropped area of the image will be the same--though of course the smaller, cropped image (or cropped area on the negative) can't make as large a print to get the same DoF effects (viewed at the same distance, with the same eyes, etc.)

If, on the other hand, we compensate for the smaller sensor size by using a shorter focal-length lens to get the same FoV, then Wrotniak's "M x A rule" applies, which implies that the smaller sensor will have somwhat greater DoF for a given aperture (at the same final print size), or that the full-frame camera's longer lens will have to be stopped down more to get the same DoF effect on the final print.

All this assuming that the apertures and degree of enlargement in question are within the limits imposed by diffraction and the pixel pitch and/or resolution of the film.
Title: Sensor size and DoF
Post by: howiesmith on October 12, 2006, 10:18:44 PM
Quote from: gkramer,Oct 12 2006, 08:05 PM
Quote from: howiesmith,Oct 12 2006, 09:15 PM

If we're comparing two images shot with the same lens at the same aperture, same subject-to camera distance, etc., then sensor size (or film size) doesn't matter, since the smaller (APC-sized) sensor amounts to a crop of the larger one, and DoF etc. within the cropped area of the image will be the same--though of course the smaller, cropped image (or cropped area on the negative) can't make as large a print to get the same DoF effects (viewed at the same distance, with the same eyes, etc.)

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That is absolutely correct.  Format doesn't matter.  

If I reduce the focal length to get the same scene onto a smaller sensor, then the focused image size will be smaller and I will need greater magnification to the final print.  Format still doesn't matter, but DoF has changed because I have a different focal length lens and degree of enlargement of the focused image.
Title: Sensor size and DoF
Post by: Ray on October 12, 2006, 10:58:01 PM
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No.  Of course not.  Surely you know better.  Does the camera format change when I change lenses (or zoom) or move about?  I can increase the tree's size in the view finder by increasing the focal length (zoom in) or move closer.  All with the same camera and its fixed format.  I am not talking about the imaging size of the sensor or film.  I am talking about the focused image size of the subject.  It is focal length and distance dependant, but not camera format dependant.

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Ray, you don't need to know the format of the camera.  You only need to be able to measure the image size in the viewer and relate that to image size in the camera.  That is format independant.

Howie,
One of us is totally confused. Let me re-phrase your statement immediately above. You measure the image size of the subject on the ground plate of your 4x5 camera. It's 1" high. You know that the relationship between the image on the film and the image on your focussing screen is 1:1. In other words, your ground glass viewer is the same size as the camera's format and the subject size on the film is also 1" high. You know, and have to know, the camera's format in order to know that relationship.

If you don't know the relationship between between the image in the viewer and the image in the camera, then the 1" measurement means nothing. Right?
Title: Sensor size and DoF
Post by: Anon E. Mouse on October 12, 2006, 11:55:15 PM
Well, whatever your personal system, the answer to the poster's question is clearly DOF does change with format size.
Title: Sensor size and DoF
Post by: howiesmith on October 13, 2006, 12:07:22 AM
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Howie,
 You know, and have to know, the camera's format in order to know that relationship.

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No.  I don't nrrd to know that.  The focused image size is the same for a given focal length lrns and ficus distance.  Trgardless of format.

The example of the same camera with an LCD diplay on the back and a laptop connection have much different image sizes from the same camera format.  It is an electronic thing.  Nothing to do with format.  IF I know a half inch image on the LCD is an inch in the camera, and a 12 inch image on the laptop is an inch in the camera, how is camera format involved.  Same camera. two different image sizes on two different viewers.


All that I have been trying to say is that if you use two cameras of different formats but with the same focal length lens, same f/stop, same CoC and same focus distance, to make equal prints (same sizw focused image), DoF is the same for bothrints.  The only variable changed is camera format.  DoF does not change.  The logical conclusion is DoF is independant of format.  Format doesn't matter.  Change one and only one variable at a time to easily and correctly see the effect of the changing variable.  Don't confuse the issue by changing moe than on variable at a time.  

It isn't that hard.  I'm thinking you are either being obstenant or putting me on.
Title: Sensor size and DoF
Post by: howiesmith on October 13, 2006, 12:08:45 AM
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Well, whatever your personal system, the answer to the poster's question is clearly DOF does change with format size.
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Wrong.  Once more:

"There was a query in October, 2001 on my Discussion Forum as to whether Depth of Field was calculated any differently for digital Vs. film. The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, ... ."  Emphasis added
Title: Sensor size and DoF
Post by: Ray on October 13, 2006, 03:54:23 AM
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It isn't that hard.  I'm thinking you are either being obstenant or putting me on.
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Well, I guess I am being a bit obstinate. I usually am when I think I'm right   .

There is a certain logic to your argument but I find it verging on the absurd. I agree, however, that one could dispense with the terminology of format and use information provided by the manufacturer, who does require knowledge of the camera's format, such as the relationship between the size of the image in the viewfinder and the image in the camera.

We could rename 'format' as being 'the maximum size crop the camera can make from the image circle of any appropriately designed lens attached to it.

You could then turn a blind eye to what that maximum size actually is on the grounds that you never use it, but I fail to see how you can make a formatless image. Whether you use the maximum format that the camera is capable of, to compose your images, or a smaller, self-imposed format, you still have to know the format.

For example, if I were continually dismayed by resolution fall-off and vignetting with my 5D (which I'm not, but I am occasionally) I could carefully draw a rectangle in the 5D viewfinder that corresponds with the FoV I would get from a 20D and paint the surrounding area black. I could then use the 5D as though it were a 20D which crops the full frame 35mm format, and all my DoF calculations and lens choices would be almost exactly the same as they would be if I were actually using a 20D, but not quite the same because by doing this foolish thing I'd have a lower resolving camera than the actual 20D, 5mp as opposed to 8mp, and the enlargement factor (or degree of interpolation required to make a print) would be greater for my modified 5D. To put it another way, the minimum size CoC I would ever use would be slightly smaller using the actual 20D, so in this sense pixel density, total pixel count and format all have a bearing on DoF.
Title: Sensor size and DoF
Post by: howiesmith on October 13, 2006, 08:40:50 AM
Ray, I am assuming that "format" is the x by y dimensions of a sensor, be it film or digital.  I think this is commonly accepted.  I don't care what happens outside those dimensions.  It isn't recorded on the sensor.  

Using that definition of format, each time I crop a negative, be it film or digital, I change the x by y dimensions of what's left or its format.  I do not in anyway change what is on the new format.  As far as the information on the new format is concerned, nothing changed.  Not the subject size, exposure or, yes Ray, the DoF.  If I make an 8x10 print, then cut it in half, the characteristics of each half are not changed.  Only the print format has changed.  The DoF of each half is unchanged.  It is so simple as to approach absurdly so.

To more easily understand the effect of changing a variable (format, focal lenth, focus distance, f/stop, etc.) it is very useful to devise tests that change only that single variable while keeping all the other constant.  For instance, it is much easier to determine the effect of changing f/stop if f/stop is the only variable I change,  If I also change camera format and lens focal length and degree of enlargement, I just might, might get confused as to which changed variable is making the change in the outcome.
Title: Sensor size and DoF
Post by: Ray on October 13, 2006, 08:45:21 AM
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Wrong.  Once more:

"There was a query in October, 2001 on my Discussion Forum as to whether Depth of Field was calculated any differently for digital Vs. film. The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, ... ."  Emphasis added
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DoF doesn't care about anything at all; neither format nor picture, nor if you are feeling well today. It's merely a conceptual tool which should be used, in fact has to be used, with format in mind to get the desired results.

Much of the time, as I wander about these ruins at Siem Reap, my camera is set on f11 in aperture priority mode. Sometimes I come across a bas relief in good condition and in interesting lighting, zoom in on it, forgetting my camera is set on f11, take the shot, realise the shutter speed was unnecessarily slow, stop up to f5.6 and retake the shot.

I've got 2 virtually identical shots. Hopefully the second shot will be marginally sharper, but maybe not, or perhaps the f11 shot will be sharper in the corners. The DoF formulas will tell me the f11 shot has greater DoF. But it hasn't. Wouldn't you agree? This is because DoF does not care about format, nor the composition, nor the picture. But I, as the photographer using the concepts of DoF, have to care about format.
Title: Sensor size and DoF
Post by: howiesmith on October 13, 2006, 10:01:20 AM
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Wouldn't you agree? ...  But I, as the photographer using the concepts of DoF, have to care about format.
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I have absolutely no idea what you are even trying to say, let alone agree with you.

If you think DoF is related to format, knock yourself out.  Keep on doing what you do.

Truth is an interesting thing, Ray.  It doesn't care whether you believe it or not.  It just goes on being true.  (I know, truth, like DoF, doesn't care about anything.)
Title: Sensor size and DoF
Post by: Ray on October 13, 2006, 10:53:36 AM
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I have absolutely no idea what you are even trying to say, let alone agree with you.
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So be it. I've done my best. Taking format into consideration works for me. I've got a glimmer of an understanding what you're talking about, I think. But I can't help associating your views with that of religious (photographic)  fundamentalism.

Ie. the basic DOF formulas do not include a format specification, therefore format has no bearing on the issue. The DoF calculators on the internet seem a little bit more progressive. To use them, you need to specify the format of your camera, by model name, to get an appropriate CoC in relation to an 8x10" print. As the great bard wrote, 'a rose is a rose by any other name'. (I've probably misquoted).
Title: Sensor size and DoF
Post by: howiesmith on October 13, 2006, 01:06:13 PM
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I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc.  But I don't know why.  Could I persuade anyone to enlighten me, please?
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macgyver,

I think your problem may be "snippets" from less than experts who fail to provide all the assumption necessary to make there position plausible.  Luckily, if you really want to know, you need not take the words of a stranger or even one trusted expert.

My opinion is DoF is not dependant on format.  Surely another "snippet," but you need  not just agree.  In fact. please don't.  Some research will convince you.  Read "Understanding Depth of Field" on this site.  You may trust Michael Reichmann.

It is my understanding that smaller format cameras have greater DoF because they usually have shorter focal lenth lenses than their full frame brothers, and not because they are smaller format.  Hope this and all the previous posts, if you could manage to actually read them, help.
Title: Sensor size and DoF
Post by: Ray on October 14, 2006, 03:18:20 AM
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It is my understanding that smaller format cameras have greater DoF because they usually have shorter focal lenth lenses than their full frame brothers, and not because they are smaller format. 
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This is the sort of circular argument that doesn't get us anywhere. I'm using a short focal length lens because my camera is a small format. The greater DoF is due to the fact that the lens has a short focal length and has nothing to do with the format.

There's a self-evident truth here if you are referring to the DoF of the aerial image projected by identical lenses used with different formats However, if you are referring to a finished print of a given size, there's a lot more going on which influences the final perception of DoF as well as the manner in which it is calculated, specifically choice of CoC.

Let's take a concrete example and compare 2 currently available cameras of different format, the new 10mp Canon G7 P&S and the older 8mp EOS 30D DSLR, both of which can use 10mm lenses.

Your argument would be, if I've understood you, that in principle a 30D image cropped to the same size as the G7 sensor would exhibit the same DoF on equal size prints, both shots taken from the same position, using not the same lens, which is not possible, but different lenses of the same focal length and at the same f stop, say f4. Is that your position?

The 30D sensor is approx 340.5 square mm in area. The G7 sensor (7.18x5.32mm) is approx 38.2 square mm. If I crop the the 30D image to the same size as the G7 image, the resulting image is less than 1mp.

The G7 lens is certainly going to be a lot sharper at f4 than the EF-S 10-22mm at f4. The 10mp G7 image at the focal plane is going to be a lot sharper than the 1mp 30D image (actually only 900kp), yet the OoF parts of both images will probably be quite close in definition or degree of blurriness.

If I make A3 size prints from both images, experience tells me the G7 print will have noticeably less DoF than the print from the heavily interpolated 30D image which will probably beging to resemble an image from a pin-hole camera.
Title: Sensor size and DoF
Post by: howiesmith on October 14, 2006, 01:13:12 PM
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... there's a lot more going on which influences the final perception of DoF as well as the manner in which it is calculated, specifically choice of CoC.

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My point exactly Ray.  A lot of factors go into DoF.  I never claimed otherwise.  Format simply is not one of those factors.

In your example, just how many variables did you change in addition to format and attribute to format?  I saw at least some pixel counts, and lens sharpness.

If you want to understand DoF, at least learn what affects it and how.
Title: Sensor size and DoF
Post by: AJSJones on October 14, 2006, 02:40:11 PM
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My point exactly Ray.  A lot of factors go into DoF.  I never claimed otherwise.  Format simply is not one of those factors.
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Ray, I do believe I understand this thread!

Howie seems very much into DoF but doesn't seem to care about the actual size of the piece of film he uses, only the dimensions of the image falling on it.  I presume film size matters to the extent that it needs to be sufficiently large for the print size he has in mind when capturing the image. (The above response derives, I think, from the fact that he often crops quite a bit, so the "original" format isn't relevant)

He therefore would not be concerned about the conventions of other people who have developed their own workflows and practices by associating them with particular format sizes and who crop (fairly) minimally or not at all (some insist that cropping of any kind after the exposure is "manipulation").  He uses CoC and enlargement factors directly to guide the design of his prints and therefore has no need of the DoF tables and calculators of others - or even the concept of "format" as used by other photographers in such discussions.  It is therefore not surprising that you and he would not communicate well about DoF and Format.

 It took me a while to understand, but I have put together a sequence of quotes that I think show that you both agree on what affects DoF and that enlargement factor and print size are critical.  (BTW I use a 6x loupe in the field with my 4x5 because I know I will not do much cropping post facto and my max print size of 24x30 represents a 6x enlargement - if it looks sufficiently sharp through the 6x I'm comfortable it will be OK on the print)

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This method works for me and my style of shooting. It probably isn't for everyone, maybe no one else. But I do get anoyed when I'm told it doesn't work or nobody does that. But then I don't photograph for anyone else but me.
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I use 4x5 and 6x6 film and can measure image sizes directly from the ground glass. No need to know the pixel size or pitch because there aren't any. If there were, I don't need to know thatanyway. (I do need to know not to try to make CoC less than the pixel size though.) Then it is a simple matter of saying that the 1 inch high tree on the ground glass will be 20 inches on the planned print. A 20X enlargement. Not hard to do and not determined by format. Note that if I am using my 4x5 and think I am using my 6x6, I still get the same results - a 20X enlargement.
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Then I divide the CoC on the print by 20 to get the value to be used in planning the in camera work. The rest is simple math using the easily determined factors like lens focal length, f/stop, focus distance, etc. It is just a matter of taking time to plan what I'm doing.
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I usually crop before enlarging.
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It is not necessary to base DoF on an average angular resolution of the human visual system. It is merely a convenience. Because DoF is personal, I can base it on anything I want, like my own visual system.

While Howie does photography his way, he does acknowledge a "common" situation where folks know the format and their desired print size and use the ratio to determine DoF considerations, with minimal (or no) cropping.  Implicitly, he acknowledges that if you change the format you would evaluate the DoF issues differently, because the ratio (of format to print)  would be different.
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Format size is useful if you plan to make only full frame enlargements. Then just ratio up the format dimension to the print dimension.

 So, "conventional" DoF tables are based on format- and print-size-based CoC's and "conventional" viewing distances, but Howie disdains those conventions and makes DoF a more personal affair.  Nothing wrong with that, but it does generate a bit of semantic confusion.

Howie, I hope I've understood your point and not misrepresented it by this.  There are many who feel that the same image falling on two different sensors can't have different DoFs, because they have not thought beyond that capture to the viewing situation.  You clearly take into consideration the properties of the print, the degree of enlargement and the viewing distance as factors that affect the appearance of the image - just a little unconventionally!
Title: Sensor size and DoF
Post by: howiesmith on October 14, 2006, 06:58:57 PM
AJSJones,

I think you have it.  DoF can be format dependant if you first assume it is and base all you printing assumptions around a particular format size.  (One can prove so many things true if you start with assuming they are true.)  Then, as I think you pointed out, the "image size" is the same as the "format size."  If that is the way you print and the way you will always wnat to print, then your assumption is fine.  I do not think it is correct to claim DoF is format dependant.

I also agree that if I make all the same assumptions made for on line DoF calculators or the stamps on the lens barrel, they will work fine, if I print and veiw to those assumptions.  If I want to be more "creative," I need to understand DoF.

Thanks for taking the time to understand this thread and your attempt to set things straight.
Title: Sensor size and DoF
Post by: AJSJones on October 14, 2006, 07:31:34 PM
Howie,

I would submit that before digital the DoF that used to be talked about pretty commonly (i.e. for 35mm cameras) was based on the convention of making an 8x12 ish sized print from the whole frame and viewing it from around the image diagonal.  It made life easier with a common reference point for workflow and discussion and didn't require a preamble discussion of CoC and enlargement factors.

The issues that are raised by the advent of digital, and this thread in particular, are a) the one of cropping either by sensor design or post facto and its effect on DoF and  the discreteness of the captured array and its relationship to CoC.  You comment that it doesn't make sense to make it smaller than a pixel; Jonathan Wienke (and others) make a sensible case for using pixel size AS the CoC while if one goes strictly by "convention" it is based solely on geometry (see above).

I wonder if folks had similar discussions on DoF as the film emulsions got better and better and resolution went beyond the "conventional" CoC  

Andy
Title: Sensor size and DoF
Post by: howiesmith on October 14, 2006, 08:09:29 PM
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Howie,

I would submit that before digital the DoF that used to be talked about pretty commonly (i.e. for 35mm cameras) was based on the convention of making an 8x12 ish sized print from the whole frame and viewing it from around the image diagonal.  It made life easier with a common reference point for workflow and discussion and didn't require a preamble discussion of CoC and enlargement factors.

The issues that are raised by the advent of digital, and this thread in particular, are a) the one of cropping either by sensor design or post facto and its effect on DoF and  the discreteness of the captured array and its relationship to CoC.  You comment that it doesn't make sense to make it smaller than a pixel; Jonathan Wienke (and others) make a sensible case for using pixel size AS the CoC while if one goes strictly by "convention" it is based solely on geometry (see above).

I wonder if folks had similar discussions on DoF as the film emulsions got better and better and resolution went beyond the "conventional" CoC   

Andy
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First, I don't own or use zoom lenses.  Therefore, filling the frame isn't always an option for me.  Cropping is frequently required.  Even if it weren't, I'm not sure I would want to make a standard print to be viewed from a standatd distance every time.

Second, I recognize that with the wide spread use of auto-everything cameras.   Many photogrpahers don't know what f/stop or focus distance their camera has selected for them.  Many don't know how an exposure meter works.  And umteen point exposure calculation, who could reproduce that?

Third, using the pixel diameter for CoC means to me that the photographer is going to get maximum DoF for each image.  Using a fixed CoC also means the photogrpaher prints and views every image the same way.  If that is an assumption you can live with, use it.  Just know and understand what you are doing, and don't simply tell every body that is it.

Lastly, did anyone worry about a CoC smaller than film could resolve and opt to call that "the" CoC?  Not that I ever heard of.

But that's just me, and my old fashioned ideas.  Thanks again AJSJones for your time, effort and insight.
Title: Sensor size and DoF
Post by: gkramer on October 14, 2006, 09:19:18 PM
The statistician John Tukey, commenting on a pre-web blog-equivalent on a statistical technique called "factor analyis," in which enthusaiasts of the technique sought to extract an entity called a "simple structure" from the data, remarked in effect that the notion of a "simple structure" was a concept more applicable to the objects doing the studying, than to the object being studied. The same might be said of the concept of a "circle of confusion," relative to this blog.
Title: Sensor size and DoF
Post by: howiesmith on October 14, 2006, 09:40:57 PM
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The statistician John Tukey, commenting on a pre-web blog-equivalent on a statistical technique called "factor analyis," in which enthusaiasts of the technique sought to extract an entity called a "simple structure" from the data, remarked in effect that the notion of a "simple structure" was a concept more applicable to the objects doing the studying, than to the object being studied. The same might be said of the concept of a "circle of confusion," relative to this blog.
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I think Tukey also said something to the effect that the price of calculation is decreasing rapidly, but the price of theorem proving is holding steady or increasing, so economics we spwmd spend more time on calculation.  So why make all those limiting assumptions to skip doing calculations?
Title: Sensor size and DoF
Post by: Ray on October 15, 2006, 01:44:17 AM
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In your example, just how many variables did you change in addition to format and attribute to format?  I saw at least some pixel counts, and lens sharpness.
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In my example I changed no variables. I didn't even change the effective format. There are differences built into the 2 cameras which are necessary design features flowing directly from the demands of different sensor sizes, ie. different formats. Smaller sensors generally have greater pixel density and require higher resolving lenses. These 2 factors alone will change the DoF outcome.
Title: Sensor size and DoF
Post by: Ray on October 15, 2006, 02:37:03 AM
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Ray, I do believe I understand this thread!

Howie seems very much into DoF but doesn't seem to care about the actual size of the piece of film he uses, only the dimensions of the image falling on it.  I presume film size matters to the extent that it needs to be sufficiently large for the print size he has in mind when capturing the image. (The above response derives, I think, from the fact that he often crops quite a bit, so the "original" format isn't relevant)

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Andy,
If you read the thread again I think you'll find I also understand this approach of Howie's and I have addressed the fallacy I see in his reasoning when he uses this approach as a basis for the statement that format is unattached to DoF considerations. I don't see how it is possible to measure the size of an image, which is a subset of the camera's format, without knowing the format. Howie has already stated that he needs to know the relationship between the image size on his viewfinder and the image size inside the camera. Knowing that relationship, which is 1:1 in the case of his 4x5 camera, he knows the format of his camera, even if he pretends not to know it.

The format of the camera is inextricably a part of those 2 essential factors, the ratio of viewfinder image to camera image and the measurement of that image in the viewfinder.

I don't see it as useful or insightful to claim that a 6x7cm back attached to a 4x5" format camera effectively turns that camera into a 6x7cm format camera. Of course it does. That's self evident. Whatever crop Howie makes from his 4x5 format camera, that becomes the new format of his camera for DoF purposes, just as it would if he attached a back to his camera holding a piece of film the exact dimensions of his crop.
Title: Sensor size and DoF
Post by: howiesmith on October 15, 2006, 10:37:38 AM
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Andy,
Knowing that relationship, which is 1:1 in the case of his 4x5 camera, he knows the format of his camera, even if he pretends not to know it.

I don't see it as useful or insightful to claim that a 6x7cm back attached to a 4x5" format camera effectively turns that camera into a 6x7cm format camera. Of course it does. That's self evident.

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Same camera.  Toyo Robo.  Image size on the ground glass is 1:1.  I am using 4x5 format film so its a 4x5.  I use 6x7 film so it's a 6x7.  So far, so good, ok Ray.  I know (but don't need to know) the format.

I use a ?x? back so it's an ?x? camera.  Ooops.  I don't know the format now.  I guess know that the image is still 1:1 won't be of any use.  I'm dead in the water until someone tells me ?x?, huh Ray.

Same is true with my 6x6.  The 1:1 doesn't change (but format does) when someone sneaks a 645, or 70mm (or whatever other formats are made for it) back on.  How do I do this now Ray?  I don't know the format.

I took a picture with my 4x5 camera.  I don't need all that image, so I take some scissors and make it into a 4x3 format.  Did I change the DoF?  No, not if I make the same largement of the remaining image.  I changed the format tough.  How'd that work Ray?
Title: Sensor size and DoF
Post by: howiesmith on October 15, 2006, 11:03:07 AM
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In my example I changed no variables. ... There are differences built into the 2 cameras ... .
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This could be part of the problem Ray.  You don't when you are changing a variable.

When I use my 4x5 and switch from Plus-X to Tri-X film, I am changing a variable.  When I process Tri-X plus 1 instead of normal, I am changing a variable.  A variable is something that can be changed.  "Changing a variable" is when I change something that can be changed.  Changing variables one at a time is when I change only one variable at a time and keep all the rest the same.  I need to be careful to recognize the variables so when I change one, I don't unwittingly change others.
Title: Sensor size and DoF
Post by: Ray on October 15, 2006, 11:16:10 AM
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I use a ?x? back so it's an ?x? camera.  Ooops.  I don't know the format now.  I guess know that the image is still 1:1 won't be of any use.  I'm dead in the water until someone tells me ?x?, huh Ray.

Same is true with my 6x6.  The 1:1 doesn't change (but format does) when someone sneaks a 645, or 70mm (or whatever other formats are made for it) back on.  How do I do this now Ray?  I don't know the format.
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Well, Howie, I can see you might have a problem there if there's no indication in the viewfinder as to what size back is attached to your camera. I've never used a large format camera, but if it's possible for someone to 'sneak' a 645 back on your 4x5, then I guess it's possible for you to forget that a 645 back just happens to be there and you might end up, either way, producing a DoF adjustment for the wrong format, thus demonstrating that DoF is format dependent.

In this situation, if you don't know your format, you not only get the DoF wrong but the composition also. To argue that the DoF is right and it's just the composition that's wrong is nonsense.
Title: Sensor size and DoF
Post by: howiesmith on October 15, 2006, 11:25:01 AM
Quote
To argue that the DoF is right and it's just the composition that's wrong is nonsense.

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No Ray, to try to reason with you is nonsense.  I'm done.
Title: Sensor size and DoF
Post by: howiesmith on October 15, 2006, 12:29:12 PM
BJL, way back on July 9, you posted:

"1) If you take photos from the same distance and compare DOF on equal sized prints viewed from an equal distance, the DOF will be the same for any combination of focal length f and f-stop N that gives the same effective aperture diameter, A=f/N. For example, f=50mm, f/1.4, f=70mm f/2 and f=100mm f/2.8 all have effective aperture diameter about A=35mm, and so all give about equal DOF in a "same print size, same distance" comparison.

"In other words, adjust f-stop in proportion to focal length to get equal DOF."

While true, when you changed format, you also changed the degree of enlargement to get your equal sized prints.  To compensate for that, you then changed focal length.  To compensate for that, you changed f/stop.  You changed much more than format.

When you crop the format, also crop the print so only format changes.  Change one and only one variable at a time to see clearly what the effect of changing that variable is.  If you do that, I think you will find DoF is not affected by format.
Title: Sensor size and DoF
Post by: AJSJones on October 15, 2006, 03:47:12 PM
Ray, I still think it's largely semantic confusion!

Howie sees image capture size as a continuous variable, given that he crops frequently and maybe a significant amount.  IOW he does not see a 4x5 sheet of film as a 4x5 "fixed format", because he may only use part of what's on the ground glass (no zooms!)  If the image to be captured were the right size for a 6x7 roll back, but he only had brought 4x5 film holders to capture it with (and threw away the rest of the image later), he needs to consider DoF issues based on the size of the image he plans to capture and the print he envisions.  In this case, you or I might look up (or use a DoF calculator) the DoF data under the heading of 6x7 ; at least I would, even knowing that it is "really" a 4x5 format camera.  Following my earlier example, if I wanted to make a 24x30 print from only part of the image on my 4x5 ground glass (already had my longest lens and couldn't get closer), I would need to use a more powerful loupe to examine the critical parts of the image because it was going to be enlarged more than my "standard".

Many out there, including you and me, have "quantized" the image capture size continuum into a discrete set of different "formats" and mentally work with them in tabular form, and assume for some of the tables, a fixed size print viewed from a defined distance.  So, for a 35mm FF situation, we can be well guided by the DoF markings on a prime lens.  However, if we plan to make a larger print to be viewed from the fixed distance (the one used to calculate the DoF scale), we would need to make some adjustment if we wanted to exercise strict control over the DoF perceived when the image is viewed.  Similarly if we want to crop the captured image and make a print of the "standard" dimensions viewed from the "standard" distance, we'd similarly have to make an adjustment.   When I'm trying to photograph smallbirds with my 500+1.4x on my 5D and they're not filling the frame, I'll try to stop down to get more DoF than "necessary" because I hope to make a "standard" size print from an image that's been enlarged more than the "standard" amount. (Tough, since I'm trying to shorten shutterspeed too, but you get my point  )

It may be a little idiosyncratic (or old-fashioned as he calls it) but if degree of enlargement and viewing distance are correctly accounted for, along with the other variables, in his workflow, where's the fallacy?

Andy
Title: Sensor size and DoF
Post by: howiesmith on October 15, 2006, 05:35:29 PM
Quote
Howie sees image capture size as a continuous variable, given that he crops frequently and maybe a significant amount.  ...

In this case, you or I might look up (or use a DoF calculator) the DoF data under the heading of 6x7 ; at least I would, even knowing that it is "really" a 4x5 format camera.  ), ...

It may be a little idiosyncratic (or old-fashioned as he calls it) but if degree of enlargement and viewing distance are correctly accounted for, along with the other variables, in his workflow, where's the fallacy?

Andy
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AJSJones,

Not quite.  "Image capture size" (format?) isn't a variable at all.  (I think that is where we started.)  It has no input to my application.  The only time I use "format" is when selecting a camera to use today, like what pants will I wear today?  The only requirement for format is whether the planned image will fit onto the film.  I seldom try to use my 6x6 with a 150mm lens to capture the width of the Grand Canyon.  (Actually, I don't think I have a camera and lens combo that will do that.)  Then, I look at the Grand Canyon, and move on.  I know I can't capture every moment, so I don't even try.

What do you do when the the cropped "negative" is, say 7.2x8.2?  Do you have a table for that or does your calculator ask for that input?  Or do you pick some cook book value that is "close enough?"  That is an odd format to say the least.

Your last statement is what I really see as the crux of this dicussion.  When using the markings on the lens barrel, tables, on-line calculators. CoC=pixel size, etc., the major factors are not usually taken into account correctly.  Many photographers just use the lens barrel DoF calculator, then make a 30x40 print.  Or worse, look at the image at 200% in Photoshop on their computer monitor viewed from 20".  Not even close to an 8x10 from 20".

I have wondered how photographers make a 3x5 post card to be viewed from 20" and a 16x20 coffee table book to be viewed from 20" and a 30x40 print to be hung on the wall and viewed from 5', all from the same negative.  And they expect the DoF to look the same in each one.
Title: Sensor size and DoF
Post by: AJSJones on October 15, 2006, 07:31:30 PM
Howie,

By "image capture size" I was trying to convey  the variable area of the ground glass that you actually end up using in the final print (you use an example of a 7.2 x 8.2 - that is the part of the image you were wanting to capture, and the rest of the image was discarded - so it doesn't matter how big the film was that contained the 7.2 x 8.2) specifically to distinguish it from a fixed format.

As for the DoF for "weird" sizes, I go back to the workflow that many, possibly most, people follow: that is to frame the composition to fill the frame at the moment of capture by moving the camera or using a different focal length.  That is why the tables and scales are popular  

While I strive for everything to be in focus in my large format landscapes, so DoF is only a concern to get everything within it (my loupe does that on the spot), I think you're being too general about photographers with your last comment   Most people will view a postcard from closer than 20", so your scenario is tending towards the "viewed from a distance of the diagonal dimension of the image" where the DoF does indeed remain the same

Andy
Title: Sensor size and DoF
Post by: howiesmith on October 15, 2006, 08:16:08 PM
OK Andy, I see your "captured image size.

OK, folks don't look at post cards for 20".  What then?  Do you still expect the same negative to produce the same DoF for post cards, coffee table books and large wall hangings?  I know the photographer has no control over his aydience, but do you honestly design a photo for post cards, books and large prints from a single image?  What do you do then the editor/publisher tells you you get a quarter page and not the accross the gutter double?  Make a note to the reader to move in?

I don't care what many or most people think.  The truth is the truth regardless of who or how many believe it.  DoF is not a popularity contest.  I have no problem with streamlining DoF by making reasonable assumptions.  The user should know what he is doing and why, before just jumping in with the lens barrel scale and then maybe wondering what went wrong.

I sometimes wonder how many folks actually plan an image, or just point and shoot.  Those who claim to take 10,000 images in a week or two, or even 100,00 in a year, don't spend much time planning.  It is my opinion that much of that is point, shoot, bracket like crazy, run on to the next spot, and edit later.  A style yes, but not mine.  I object to "that's how everyone does it."
Title: Sensor size and DoF
Post by: Ray on October 16, 2006, 01:09:59 AM
Here's a shot I took using the HFI method of DoF calculation ('Howie Format Independent' method). I borrowed a 4x5 format camera with a 150mm lens. Based my DoF calculations on the size of the dancer on the ground glass plate and I think I got the background with just the right amount of blur, enough not to distract from the main subject but not so blurred as to be unrecognisable.

What a pity I was unaware the camera had been fitted with a 6x7cm back. The DoF is just fine, though   .

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Title: Sensor size and DoF
Post by: Craig Arnold on October 16, 2006, 05:06:32 AM
Ray is right and Howie is wrong.

Howie simply isn't using the commonly accepted definition of DOF.

I think the notion of HFI DOF is at the crux of the matter. You can call a horse a donkey and then debate about how your donkey runs faster, and looks better and is taller than all the other donkeys. But the problem is that you have misunderstood the definition of donkey.

COC is dependent on film format.

The DOF calculations use COC as a variable.

When you change format you have changed your COC, and so you will change DOF.

Wiki entry - Circle of Confusion (http://en.wikipedia.org/wiki/Circle_of_confusion)

Wiki entry - Depth of Field (http://en.wikipedia.org/wiki/Depth_of_field#DOF_limits_and_hyperfocal_distance)

Wiki extracts:

In film photography, the circle of confusion criterion is sometimes defined as the largest circle of blur on a film negative that will still be perceived by the human eye as a clean point when printed at 30 cm diagonal size and viewed from a normal viewing distance of 50 cm (and variations thereon).


Accepted values for circle of confusion based on d/1500
Film format Frame size[1] CoC

Small Format
APS-C[2] 22.5 mm x 15.0 mm 0.018 mm
35 mm 36 mm x 24 mm 0.029 mm

Medium Format
645 (6x4.5) 56 mm x 42 mm 0.047 mm
6x6 56 mm x 56 mm 0.053 mm
6x7 56 mm x 69 mm 0.059 mm
6x9 56 mm x 84 mm 0.067 mm
6x12 56 mm x 112 mm 0.083 mm
6x17 56 mm x 168 mm 0.12 mm

Large Format
4x5 102 mm x 127 mm 0.11 mm
5x7 127 mm x 178 mm 0.15 mm
8x10 203 mm x 254 mm 0.22 mm


Of course these values are for d/1500. If your lenses and film/sensor have higher resolution and you vary print sizes and viewing conditions etc you might want to change the value, but for whatever values you use you can draw up a similar chart for COC of different formats.



A list of my favourite Howie quotes from this thread:

(Taken somewhat out of context for humorous effect   )

One can prove so many things true if you start with assuming they are true.

A variable is something that can be changed. "Changing a variable" is when I change something that can be changed.

"Image capture size" (format?) isn't a variable at all.

I don't care what many or most people think. The truth is the truth regardless of who or how many believe it. DoF is not a popularity contest.

Because I don't own a pixel, I need not know anything about its size.

I have not yet run into a case where my CoC was smaller than the lens/film resolution. In fact, I don't even know what that is for my lenses and film selections.

All that I have been trying to say is that if you use two cameras of different formats but with the same focal length lens, same f/stop, same CoC and same focus distance, to make equal prints (same sizw focused image), DoF is the same for bothrints. The only variable changed is camera format. DoF does not change. The logical conclusion is DoF is independant of format.

It isn't that hard. I'm thinking you are either being obstenant or putting me on.

If I make an 8x10 print, then cut it in half, the characteristics of each half are not changed. Only the print format has changed. The DoF of each half is unchanged. It is so simple as to approach absurdly so.
Title: Sensor size and DoF
Post by: howiesmith on October 16, 2006, 10:13:57 AM
peripatrtic,

"[Y]ou can't have a DOF number without a COF number, and the COF number is one decided on by you or the lens manufacturer, whomever you trust the most."  Emphasis added

To "lens manufacturer," I would also add Wiki definition, peripatetic, or Ray.  I still trust myself.  Again, if you first assume DoF is format dependant, it is easy to prove it.

Try tis out.  I plan an image with my 4x5 camera with 320mm lens.  I set the CoC at d/1500.  The image plan calls for a 3x5 format, so CoC is 0.0039".  My plan is to make a 6x10 print for my portfolio book.

I do that.  My wife sees the print and likes the image, so suggests it would make a nice 3x5 post card.  So I print up a few.  I mail one to the editor of my next book, who wants to include it as a 12x20.  My neighbor sees the book and wants a 48x80 print for his wall.  Do all these prints have the same DoF?  If CoC=0.0039 is correct, I would say yse, until I look at each print.

I assume Ray's example was meant to be more sarcastic than instructive.  I serious doubt the iamge shown was taken with a 4x5 camera with a 6x7 back.  My hunch is some sort of digital that Ray owns, maybe a Canon, with a zoom lens.  I doubt Ray even know what focal length the lens was set at, the focus distance or the f/stop.  The focus distance and f/stop were probably set by his Canon.  ANyway, if the planning were actually done, the plan would alos include a print size, giving enlargement from a 4x5.  But alas, poor Ray used a 6x7 instead with his 4x5 plan and got the sorry image shown in his example.  If Ray were to continue with the plan and make a print of the 6x7 negative enlarged the same degree as the planned 4x5, he would find the DoF on his print was as planned.  I think that would prove DoF is not format dependant.  Two different formats treated the same produced the same results.  Whether the planned image will fit into the selected format is format, lens focal length and focus distance dependant, and that part of the planning has nothing to do with DoF planning.  I would guess that Ray does very little to no image planning and that may be why he does not know what is involved.  (He has claimed before that he doesn't have time for planning because he is too busy running around trying to capture the moment.)
Title: Sensor size and DoF
Post by: Craig Arnold on October 16, 2006, 10:42:45 AM
Quote
peripatrtic,

I do that.  My wife sees the print and likes the image, so suggests it would make a nice 3x5 post card.  So I print up a few.  I mail one to the editor of my next book, who wants to include it as a 12x20.  My neighbor sees the book and wants a 48x80 print for his wall. 

OK good example.

Quote
Do all these prints have the same DoF?  If CoC=0.0039 is correct, I would say yse, until I look at each print.

If one were to say yes, then that person would clearly not understand what CoC means; of course they don't have the same DoF.

Start with the definition again.

Integral to the concept is the print size, and how much enlargement of the negative is required. The standard used is a 30cm print and viewing distance of 50cm. Both are integral. DoF changes if you move back a few paces too.

But it is crucial that 30cm size and 50cm viewing distance are a baseline. It doesn't really matter what the baseline is set to, but without one we are just chasing our tails.

You can define the baseline print size and viewing distance wherever you like, but without them you have no point of reference and the formulae become useless.

In practice of course you (and most everyone here) has a very good grasp of how to achieve the required DoF for their photographs. And considering the equipment you use I have little doubt that you are a great deal better at it than most small-format users (digital or not). You simply have to be in order to get the results you require.

However I think your practical knowledge has mislead you into thinking you have the theoretical concepts down pat. And you are simply mistaken. CoC and DoF simply do not mean what you thought they did.

It all comes down to how you answer the question: When you crop a shot do you change the DoF?

Your answer: "No of course not". Correct answer: "Yes, obviously."

Because inherent in the notion of that CoC from which DoF is calculated is that you are viewing a 30cm print from 50cm distance. So by cropping it is assumed that you mean crop and enlarge to a 30cm print viewed from 50cm.

It isn't even meaningful to compare DoF with prints that differ in size and viewing distance.
Title: Sensor size and DoF
Post by: howiesmith on October 16, 2006, 11:06:21 AM
"If one were to say yes, then that person would clearly not understand what CoC means; of course they don't have the same DoF."  They all have the same format and CoC don't they?  Only the degree of enlargement changed.  It is the degree of enlargement that matters here, not format.

"Integral to the concept is the print size, and how much enlargement of the negative is required. The standard used is a 30cm print and viewing distance of 50cm. Both are integral. DoF changes if you move back a few paces too."

You are assuming a 30cm print viewed at 50cm.  You don't need to do that.  DoF can be taylored to any print size and/or viewing distance.

Think of the CoC on the negative as a subject.  How much can you enlarge that subject before it starts to look like a disc instead of a point on your print?  Your print, not just an arbitrary 30cm print.  At any viewing distance you select, not just an arbitrary 50 cm.

"But it is crucial that 30cm size and 50cm viewing distance are a baseline. It doesn't really matter what the baseline is set to, but without one we are just chasing our tails."

It is not only not crucial, it isn't even necessary.  I can select any CoC I want, any print size I want and any viewing distance I want.  To quote someone else, "I don't need no stinking [standard]."  And I don't even want one.

You say I can start with any standard.  If I can do that, I can just skip the standard and go on.  How do you make a 50cm print to be viewed at 60cm?
Title: Sensor size and DoF
Post by: howiesmith on October 16, 2006, 02:22:47 PM
peripatrtic,

The reason I do not need a standard is all the work that goes into making a standard as a place to start is the same as I do already.  Essentially, I create a new standard from a non-format specific base for my own use.  That standard happens to be the print size and DoF I want.  No adjustments required.  If I used a standard, I would have to do the same or perhaps even more work, to adjust the standard to what I need.  So I figure , why bother?  Just start from the basics that I understand and go to my specific, making whatever assumption I want (and know are being made).

If I wanted to make a 30cm print to be viewed from 50cm, I can work that out without using to camera format - just the degree of enlargement from the negative to my print.  How much do I want to enlarge the CoC on the negative to get the desired CoC on the print.

I don't think this is wrong and I know I don't need to know or use the camera's format.  Camera format could be useful if I knew it, wanted to make an uncropped (full frame) print.  I then know the degree of enlargement of the negative is equal to the degree of enlargement of the film's CoC or subject image.  Note that the degree of enlargement of the full frame (format) negative is not usually the same as the degree of enlargement of the subject image. They are equal when and only when no cropping is being done.  This is a very specific case and not the general rule.
Title: Sensor size and DoF
Post by: Craig Arnold on October 17, 2006, 03:18:50 AM
That's cool.

As I said, you obviously know what you need to do to get the results you want.

But when you say depth-of-field, you are not using the standard definitions, hence all the confusion.

Quote
They all have the same format and CoC don't they? Only the degree of enlargement changed. It is the degree of enlargement that matters here, not format.

If you change the enlargement and keep the viewing distance the same you have changed the CoC. That is how CoC is defined.

Your "Howie Format Independent" understanding of the concept is different from the accepted definitions. That's fine if it works for you, but you need to understand that most people will work with the standard definitions, and I don't think it's really fair to berate them for that.

Once we all know that you mean something slightly different and understand what it is then we can understand what you mean.  

Quote
You say I can start with any standard. If I can do that, I can just skip the standard and go on. How do you make a 50cm print to be viewed at 60cm?

I believe what they did in the first place at Leica or Zeiss or somewhere (in fact I think the Wiki article includes a copy of some of the early research) is sit down a bunch of people and ask them whether pictures  enlarged to different degrees were sharp. So to create another baseline you either interpolate (perhaps there are some standard formulae) or you experiment by sitting down a bunch of people and asking them, or you experiment with yourself as the viewer and gather up experience over a number of years. You then either lay the results down formally or use your neural net to allow you to estimate it pretty accurately without any formalisation.

The latter method is very effective for producing individual results, but not much use for teaching or passing on that knowledge to other people in the industry.

At any rate you are determined to call your horse a donkey, and it's clear both will get you where you want to go.
Title: Sensor size and DoF
Post by: howiesmith on October 17, 2006, 10:07:38 AM
If you insist that DoF is format dependant becasue Wiki told you it is, well then, it is, for you anyway.  Wiki is your undisputed source.  You seem to trust it more than yourself.  I guess that before Wiki, there was no DoF.

I use as my source a book by Ansel Adams.  He provided a write up on DoF using CoC that wasn't bounded by camera format or a 30cm print.  But he wrote the book before digital cameras and Wiki came along.  He was wrong too and died not even knowing it.  Too bad.
Title: Sensor size and DoF
Post by: Ray on October 17, 2006, 12:46:48 PM
Quote
I use as my source a book by Ansel Adams.  He provided a write up on DoF using CoC that wasn't bounded by camera format or a 30cm print.  But he wrote the book before digital cameras and Wiki came along.  He was wrong too and died not even knowing it.  Too bad.
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Well that explains it, Howie. You should have told us sooner that you are not referring to DoF in a picture, or on a print, which as you know must be a specified format and size. I understand the principle that you could measure the size of a particular object within a composition, in the viewfinder (although that's rather difficult with an SLR), and determine what CoC is required if that object is to appear sharp when enlarged, say 8x on a print. You could then measure another object further away, which is smaller and doesn't require the same resolution of fine detail to appear sharp, and then measure the size of an even smaller object even further away, or a small object in the foreground. By choosing a combination of lens focal length, distance to main subject and f stop, you could determine that all the essential ingredients in your composition are going to appear as sharp as your eye can see (or not, whatever the plan is) when enlarged by a certain degree and viewed from a certain distance.

All this might be possible without reference to a precise format before the shot is taken. You have a composition within an imaginary format of at least a minimum size to include the ingredients of the composition. Your calculations cannot be unattached to a format if you want to take the picture and make the print. If they were you would have no guarantee that the image would not turn out like my example of the dancer at Angkor Wat.

Of course I didn't really borrow a 4x5 camera. The shot was taken with a 5D and 50/1.4 lens at f2.8. I cropped it with a 6x7 aspect ratio to demonstrate the sort of thing (and worse) that could happen when DoF is calculated without reference to format. In that example, if I'd been aware that the camera I was using was really a 6x7cm format despite the viewfinder being 4x5, I would have needed to step back a few paces to fit the ingredients of the composition within the 6x7 format and increase the aperture a stop or so up to maintain that same DoF.

There's absolutely no point in measuring objects and subjects within your composition if you don't know that they will fit on the piece of film in the camera.

The fact that it is difficult to not know the format of your camera and difficult to be uncertain that your composition in the viewfinder will fit on the sensor, has no bearing on the matter. Whether you know your camera's format through habit without thinking about it, or whether the manufacturer has designed a fool proof system that does not allow you to measure objects that will not fit on the sensor or film, does not detract from the absolute necessity of knowing  the camera format (or your own modified format) in order to make appropriate adjustments to lens and subject distance for DoF purposes on a specified print size.
Title: Sensor size and DoF
Post by: AJSJones on October 17, 2006, 01:52:39 PM
Quote
.... the absolute necessity of knowing  the camera format (or your own modified format) in order to make appropriate adjustments to lens and subject distance for DoF purposes on a specified print size.
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Peripatetic, The definition of DoF that all agree on is " the range of distances in front of and behind the focus plane that appear sharp (or in focus)".  THEN, convention takes over to calculate the DoF for (arbitrarily decided) print size and viewing distance.  Once people adopt the convention, they can talk to each other without having to go through the "print size and viewing distance" discussion and assumptions.  The 30 cm print and 50cm viewing distance is NOT part of the above definition , it is a convention and it applies only to those conditions. Those who know what variables affect their specific outcome will work based on that convention to  e.g. increase or decrease their aperture, because they plan to deviate from those conditions.  It's a guide they know how to use.  If I make a 50 cm print and view it from 30 cm , am I doing something wrong or just different?  The DoF will not be the same as the 30 cm print viewed from 50 cm, so I need to adjust the acquisition parameters to achieve my goal.  What DoF is is defined above.  How you calculate it depends on the situation.  The 30cm print viewed at 50cm is just one example and the DoF tables/scales are correct BUT only for that situation.

Ray, Is the format you speak of the ones adopted by convention/availability (645, 6x7 4x5 etc) or the actual size of the image on the ground glass that ends up being used in the print?  You have acknowledged that Howie might have a "modified" format and may choose to specify his print size differently.  But with the "phenomenological" definition above (not specified to format/print size, but just appearance of sharpness) I think Howie is doing the right thing to achieve his goal of having some things appear in focus and some things out of focus in his print based on the (artistically but arbitrarily selected dimensions of the) image he captured.  

In his example, if he captures a 3x4 image on a 4x5 or an 8x10 sheet of film and plans to make a 15x20 print from it and view it from 50 cm, what dimensions does he use in the calculations?  3x4, 4x5 or 8x10?????

How do you deal with DoF when you crop heavily???
Title: Sensor size and DoF
Post by: howiesmith on October 17, 2006, 07:32:07 PM
Quote
Peripatetic, The definition of DoF that all agree on is " the range of distances in front of and behind the focus plane that appear sharp (or in focus)".  THEN, convention takes over to calculate the DoF for (arbitrarily decided) print size and viewing distance.  Once people adopt the convention, they can talk to each other without having to go through the "print size and viewing distance" discussion and assumptions.  The 30 cm print and 50cm viewing distance is NOT part of the above definition , it is a convention and it applies only to those conditions. Those who know what variables affect their specific outcome will work based on that convention to  e.g. increase or decrease their aperture, because they plan to deviate from those conditions.  It's a guide they know how to use.  If I make a 50 cm print and view it from 30 cm , am I doing something wrong or just different?  The DoF will not be the same as the 30 cm print viewed from 50 cm, so I need to adjust the acquisition parameters to achieve my goal.  What DoF is is defined above.  How you calculate it depends on the situation.  The 30cm print viewed at 50cm is just one example and the DoF tables/scales are correct BUT only for that situation.

Ray, Is the format you speak of the ones adopted by convention/availability (645, 6x7 4x5 etc) or the actual size of the image on the ground glass that ends up being used in the print?  You have acknowledged that Howie might have a "modified" format and may choose to specify his print size differently.  But with the "phenomenological" definition above (not specified to format/print size, but just appearance of sharpness) I think Howie is doing the right thing to achieve his goal of having some things appear in focus and some things out of focus in his print based on the (artistically but arbitrarily selected dimensions of the) image he captured. 

In his example, if he captures a 3x4 image on a 4x5 or an 8x10 sheet of film and plans to make a 15x20 print from it and view it from 50 cm, what dimensions does he use in the calculations?  3x4, 4x5 or 8x10?????

How do you deal with DoF when you crop heavily???
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I think that clarifies my thoughts.

Actually it is very easy for me to measure the size of the image on the ground glass.  I use a ruler and a pair of dividers.  I put the points of the dividers on the top and bottom of what I want to measure, then transfer to the ruler.  On my 4x5, the ground glass is right there on the back of the camera, easy to reach.  On my 6x6 (an SLR), the ground glass is right there on the top.  I need to have the prism and the waist level magnifier removed to reach it easily, but that is how I use it.

The ability for my camera to capture an image on the film as intended has no relationship to DoF.  Yes, it is very possible to have a poor composition with perfect DoF.  I have accomplished that many times.  It's called a reshoot.  DoF and composition are truely unrelated.  I certainly don't need to know anything my composition to calculate DoF.  If I need to change the lens or move my camera to get the right composition, then I also need to redo the DoF calculation.  I have changed the image design.  The lens' focal length and/or focus distance have changed.  I think that is pretty fundamental and should be obvious to even the most casual reader, regardless of how they do DoF.
Title: Sensor size and DoF
Post by: Ray on October 18, 2006, 12:01:38 AM
Quote
In his example, if he captures a 3x4 image on a 4x5 or an 8x10 sheet of film and plans to make a 15x20 print from it and view it from 50 cm, what dimensions does he use in the calculations?  3x4, 4x5 or 8x10?????

How do you deal with DoF when you crop heavily???
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Andy,
If Howard captures a 3x4 image on his 4x5 viewfinder of his 4x5 camera (or captures a 4x5 image with the intention of later cropping it to 3x4), then for DoF purposes, whatever the subjective variables of viewing distance and eyesight capability, the results will be exactly the same as though he had used a 3x4 format camera without cropping, using the same lens, same distance, same f stop. He knows the format because he has measured it. It is disingenuous of Howard to claim that the format has no bearing on his DoF calculations.

Howard seems to be claiming that he hasn't measured the format but just the size of an object, or objects, within an imaginary or planned format. Whether the format is is in the planning stage, or undecided at the time the shot is taken and created later in post processing, the format is the reference point for DoF purposes.

How do you deal with DoF when you crop heavily? You've just effectively converted your camera into a very small format camera using probably a rather low resolution lens in relation to what's available for small formats.

If the lens, film or sensor are of the same quality, you simply get the DoF characteristics that would apply using a dedicated small format camera with the same lens and settings. For example, if I use a 1Ds2 with a 50mm lens at f8, then decide later to crop the image to the same size as I would have got using a D60 with the same lens at the same f stop from the same position, I get the same image in all respects.

The point here is that for DoF purposes it is the format of the image that is actually used in the process of making a print that has a direct bearing on the final DoF perception.
Title: Sensor size and DoF
Post by: Ray on October 18, 2006, 12:51:14 AM
On the matter of cropping heavily, the DoF formulas will give you only very approximate results because they don't take into consideration such factors as lens quality and film or sensor resolution, ie. system resolution. These factors play no part when cropping moderately as in the case of the 1Ds2 image cropped to the size of a D60 image, but do play a significant part with extreme cropping.

For example, one of Howie's analogies is an 8x10 view camera with standard 300mm lens but using a 35mm format back. The formulas tell you that DoF will be the same as using a standard 35mm camera with 300mm lens at the same f stop and same distance to a central object within the composition. I doubt whether any 35mm lens would have a sufficiently small aperture setting that would be required to get equal DoF in these 2 situations. In the aperture ranges that are typically used with a 35mm format 300mm lens, the lens will be very much sharper than a standard 300mm lens for 8x10 format. The 35mm image from the dedicated 35mm camera will thus have less DoF, at the same f stop, to the extent that the lens is sharper.

I would expect the differences to be even greater in my example of a Canon 30D image cropped to the same size as a G7 P&S camera which uses not only a sharper lens than the DSLR equivalent (EF-S 10-22) but also a much higher resolving sensor. But I'm repeating myself.  
Title: Sensor size and DoF
Post by: Craig Arnold on October 18, 2006, 03:25:10 AM
Quote
Peripatetic, The definition of DoF that all agree on is " the range of distances in front of and behind the focus plane that appear sharp (or in focus)". THEN, convention takes over to calculate the DoF for (arbitrarily decided) print size and viewing distance. Once people adopt the convention, they can talk to each other without having to go through the "print size and viewing distance" discussion and assumptions. The 30 cm print and 50cm viewing distance is NOT part of the above definition , it is a convention and it applies only to those conditions. Those who know what variables affect their specific outcome will work based on that convention to e.g. increase or decrease their aperture, because they plan to deviate from those conditions. It's a guide they know how to use. If I make a 50 cm print and view it from 30 cm , am I doing something wrong or just different? The DoF will not be the same as the 30 cm print viewed from 50 cm, so I need to adjust the acquisition parameters to achieve my goal. What DoF is is defined above. How you calculate it depends on the situation. The 30cm print viewed at 50cm is just one example and the DoF tables/scales are correct BUT only for that situation.

Quite so, and eloquently put. But in order to calculate some useful values for any given format, one must (arbitrarily) choose a print size and viewing distance. Without that it becomes impossible to gather the evidence (what actually does look sharp to people) and apply the formulae.

Of course, given sufficient input, our neural nets are quite adept at adjusting for all the variables simultaneously, and ending up with the required results.

The problem with the "experience" approach is that it's very difficult to impart that knowledge except by saying "you will learn eventually if you practice enough". It can also be extremely difficult to disentangle the actual variables taken into account unconsciously.

I think the point that Ray is making so well is that Howie is in fact taking format into account no matter how vigorously he denies it.

By analogy, most people who can hit a baseball quite well wouldn't have the vaguest idea of the mathematics behind what they are doing. They might well strenously deny that they are taking account of Newton's Law of Universal Gravitation. But they are; they have an approximation of the formulae in their heads that they have built up by hitting the ball (and living on the planet) so many times.

In fact I have a challenge for Howie - can you produce some tables for us that would allow Ray and I to borrow or rent a large format setup for a weekend and go out and have a good chance of getting the required DoF with a couple of lenses and variety of apertures, and at different print sizes and viewing distances. You may not make any reference to the format/crop.
Title: Sensor size and DoF
Post by: howiesmith on October 18, 2006, 09:55:29 AM
Quote
In fact I have a challenge for Howie - can you produce some tables for us that would allow Ray and I to borrow or rent a large format setup for a weekend and go out and have a good chance of getting the required DoF with a couple of lenses and variety of apertures, and at different print sizes and viewing distances. You may not make any reference to the format/crop.

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No, I cannot produce the tables you seek as proof because each photo is different.  Simply apply the DoF calculations for each photo at a time.

Tables imply that you have decided on a set of fixed variables (say a print size) and the table then lists some numbers as you change on variable.  "DoF tables" are possible only because the user has already arbitarily decided on a certain set of fixed values for teh finished print.  It is impossibe to make a two dimensional table that accounts for changing more than two variables.  So, the makers of tablrs fix several variables, arbitarily, so they can make a table that shows the effect of changing two variables.

In fact, your tables show only decrete values, like f/stop.  The table shows only he common, marked on the lens values.  If I want f/9.2, I interpolate.

Lens makers used to put markings on their lenses to show DoF.  These were mechanical DoF calcuators.  How would you do that for any print size, plus any print viewing distance, plus any viewer conditions, as well as any focus distance, and f/stop.  So, they assumed all prints were one certain fixed size, viewed from one certain fixed distance, one CoC fit every condition, and none of their negative would be cropped away.  These assumptions had to be made in order to account for all those variables in a "calculator" that was capable of only two inputs.  But then the photographer was stuck with a "calculator" on the lens barrel that has only focus distance and f/stop inputs for that focal length lens. Problems are:

1. these assumption just aren't usually (never?) aren't true.  I have no prints designed to be 30cm prints and viewed from 50cm and aren't from a cropped negative.  How many do you have?

2. Users forgot about how DoF really works and starting thinking the calculators were right for everything.  

3.  Users began to think the calcualtors worked for every situation.  They ignored the assumtions built into the calcuators.

4.  Finally, the calcuator was on-line.  Has to be true.

On-line calculators usually have more than two inputs.  But to produce a table using these calcuators, the user would have to fix all but two (the horizonal and verticle table vaules) in order to produde the third value to put in the tables.  You just make a two dimensional (variable, if you will) table with more than two dimensions (variables).   If you fix a CoC, then all the CoC pages are reduced to one.  One page is all that is needed for one variable.  Still too many pages (dimensions).  So fix the enlargement size.  Still too many pages (dimensions).  Fix the format so enlargement can be reduced to one page.  How about fix the camera.  One camera, one page, and enlargement means only one thing.  Can't have more than one full frame enlargement factor for one print size.  And the beat goes on.

To make the original aswer to your question more accurate, not a table, but books.  You won't carry a programmed pocket calculator, and a ruler.  Would you lug around a set of books?  No.  You would keep throwing away books by assuming your print will fix that variable.  Don't need books for all those possible print sizes.  You don't have a 400mm lens, so you don't need the 400mm books.  But a 70-200 zoom has plenty of focal lengths (books).  All those CoC values.  Too many.  Assume one size fits all.  I think you get the idea.

The answer is no, tables don't work for multidimensional problems.  DoF, like it or not, is a multidimensional problem.  DoF can be simplified by making a lot of assumptions that replace variables with constances.  But then you have to live within those assumptions.

If I had only one lens, then focal length would not be a variable for me.  If that one lens were a mirror lens, then f/stop goes away.  This is getting easier.  Why not fixed focus.  Only at the hyperfocal distance.  But you would argue that focal length, f/stop and focus distance are essential.  Why?  Because you own a zoom lens with an adjustable iris that you you (or your camera) can focus and you want to make those adjustments.  Well, I can make different sized prints.  I can crop.  So I don't limit myself to 30cm prints from full frame negatives.  (I can slice, dice and make julianne fries but that is another topic and some humor.)
Title: Sensor size and DoF
Post by: AJSJones on October 18, 2006, 03:32:09 PM
Peripatetic, I think we've been in agreement - I use DoF calculators and scales (if the lens has one) and use a reasonable CoC, and find the "convention" is extremely useful for disucssion and education.  I was just saying Howie's approach is different is all - interesting to note here that in Japanese the same word is used for "wrong" and "different" (chigau).

Ray,

"The point here is that for DoF purposes it is the format of the image that is actually used in the process of making a print that has a direct bearing on the final DoF perception. "

  I think this is what Howie has been saying all along.  This now becomes clear to me that the word format can mean different things in different contexts or conventions.  If you ask most photographers "What format do you shoot?" they would answer 35 FF, 645 6x7 8x10 etc and the common understanding is that those are the dimensions of the image acquisition area.  In the sentence I quote, you are using the word to refer to the actual image used - therein lies almost all this discussion which seems to revolve around a simple semantic issue here - if you define "format" that way, I suspect Howie would agree...
Title: Sensor size and DoF
Post by: Craig Arnold on October 18, 2006, 03:48:05 PM
Howie you make some very good points. And of course it would be totally unreasonable to expect you to produce all those books just for our edification.

As it happens I often do have a programmable calculator with me. (How sad is that?!!)

But at any rate, let me reduce the variables and hope you can still meet my challenge. This should boil down to one single line from one of those books.

The rental shop up the road tells me that they will have a 150mm lens available for use on one of their large format backs this weekend.

I have a particular scene in mind at Kew gardens. It will be a vertical shot of a tree, it has an interesting twisted trunk, and the leaves are turning colour just at the moment. The top will be approximately 30m away from me, and I would like to get the top of the tree sharp and as much of the tree in sharp focus as possible, I would like to know in advance though from what distance I may expect the trunk above me to be sharp for composition purposes and to adjust the angle from which I shoot the tree and tilt the camera.

As it happens this shot is one of a series which I have been printing to 16"x20" and I have a frame waiting for this last shot to complete the series.

It will be my first use of large format so I don't want to get into shift and tilt, I feel that is all too complicated and will have to wait until after a thread on Scheimflug.

One thing that the camera shop cannot tell me at the moment is whether they will have an 8x10 or a 4x5 back available, because another customer has priority and he will only decide on the day. I will have to take what is left over.

Of course I realise that being stuck with the 150mm lens I will simply have to take whatever angle of view I get on the day and that would differ between the two backs. However I'm confident that I shall nevertheless be able to compose the scene to my satisfaction.

As I do not have a darkroom, I shall be handing the negatives to the same shopowner, but he tells me he is not going to crop for me he will print a 16x20 from either the 4x5 or 8x10. I am fairly sure the aspect ratio will work out either way.

Perhaps you could help me with what aperture and focal distance I should set for this shot. If I understand you, it will make no difference what back I am using. I would also be interested in knowing how you choose the appropriate CoC for this shot, which of course will be completely unrelated to the back I happen to be using on the day.
Title: Sensor size and DoF
Post by: howiesmith on October 18, 2006, 04:57:49 PM
Quote
Howie you make some very good points. And of course it would be totally unreasonable to expect you to produce all those books just for our edification.

As it happens I often do have a programmable calculator with me. (How sad is that?!!)

But at any rate, let me reduce the variables and hope you can still meet my challenge. This should boil down to one single line from one of those books.

The rental shop up the road tells me that they will have a 150mm lens available for use on one of their large format backs this weekend.

I have a particular scene in mind at Kew gardens. It will be a vertical shot of a tree, it has an interesting twisted trunk, and the leaves are turning colour just at the moment. The top will be approximately 30m away from me, and I would like to get the top of the tree sharp and as much of the tree in sharp focus as possible, I would like to know in advance though from what distance I may expect the trunk above me to be sharp for composition purposes and to adjust the angle from which I shoot the tree and tilt the camera.

As it happens this shot is one of a series which I have been printing to 16"x20" and I have a frame waiting for this last shot to complete the series.

It will be my first use of large format so I don't want to get into shift and tilt, I feel that is all too complicated and will have to wait until after a thread on Scheimflug.

One thing that the camera shop cannot tell me at the moment is whether they will have an 8x10 or a 4x5 back available, because another customer has priority and he will only decide on the day. I will have to take what is left over.

Of course I realise that being stuck with the 150mm lens I will simply have to take whatever angle of view I get on the day and that would differ between the two backs. However I'm confident that I shall nevertheless be able to compose the scene to my satisfaction.

As I do not have a darkroom, I shall be handing the negatives to the same shopowner, but he tells me he is not going to crop for me he will print a 16x20 from either the 4x5 or 8x10. I am fairly sure the aspect ratio will work out either way.

Perhaps you could help me with what aperture and focal distance I should set for this shot. If I understand you, it will make no difference what back I am using. I would also be interested in knowing how you choose the appropriate CoC for this shot, which of course will be completely unrelated to the back I happen to be using on the day.
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What do you use your programmable calculator for?  Is it programmed with the cook book DoF calcs or something else?

How about instead of saying you have reduced the variables, we say you have selected a few.  The degrees of freedom of this system have not changed.

4x5 or 8x10 makes no difference for DoF but you have selected a full frame enlargement of the negative.  No.  You can't do that.  You want to change the degree of enlargement in mid stream.  I don't think that's fair even in your frame of reference (all I need to know is format) if you can't decide between a post card or a bill board.  Maybe a 30cm print?

The format size determines how much of the tree is on the negative, and that has nothing to do with DoF.  If I trim an 8x10 neagtive to a 4x5, do I change anything on the negative?  Of course not.  If while you are waiting to take the photo, the tree grows another 10 meters, you still get the same portion of the tree as before, just maybe not all of it from 30m and up.  A 150mm lens forms the same sized image on any format.  If a 30m tree just fills the frame, you will still get 30m if the tree grows to 40 m.  At the risk of being pendantic, a 150mm is a 150mm lens is a 150mm lens.  The two images you have in mind, one 4x5 and one 8x10, will differ in enlargement by a factor of two if you are planning to make full frame enlargements and that does change DoF.

If you deside the tree will be so many inches high on the negative (and this a very reasonable assumption given you are going to use a 150mm for either, you are in good shape.  Format independant.  Otherwise, I'm afraid you must decide just how much you will be enlarging the portion of the tree on the negative.  You can't just deside to make a print bigger in stream.  

I know you recognize that the tree will be the same "size" on both negatives, you will just see more of it with the 8x10 back.  It is the lens focal length and focus distance that determines the tree's projected image size.  Once you select that, the focused image size is fixed.

For simplicity sake, I will assume the focus distance is 30m.  You haven't provided enough information to determine the focus distance is the camera isn't pointed directly at the top of the tree.

How much DoF do you want.  If you want all you can get, just stop down to the smallest aperture (largest f/number) you got.  We will assume the tree will be about in the middle of the DoF range, and that you don't care that half the DoF is in front of the tree and half is behind.  (30m is much much greater than 150mm.)

I will not continue.  I think you may get the idea.  You can't leave the planning of  important details like the degree of enlargement till later.  Or you will need to plan on location when certain values are available.  If you say Im' going to make a 16x20 print from either format, but the print will be the same (crop the 8x10), then you can go on.

I am also assuming that you are aware that for a 150mm lens and a subject 30m away, the focused image size is fixed and can be calculated, regardless of the film format.  That's just optics.  Ha nothing to do with the size of the screen that image is being focused on (format) or DoF.  Again, at the risk of being pendantic, a 150mm is a 150mm lens is a 150mm lens.

Let me challenge you.  You decide on the 4x5.  You have the 150mm lens.  But instead of a 30cm print, you want to make a 50cm print.  And indead of viwing it from 50cm, you want to hang it on the wall and view it from 100cm.  And you just got new glasses, so you see better, and wnat the CoC on the print to be 2/3 of the standard.  And to make life simple, let's further say the focus distance is 30m.  And you decide on f/19.3.  What is your cookbook DoF?

Like I am with Ray, I'm beginning to think you are not willing to see this or change your believe.  I feel the conversation should end with a simple let's just agree to disagree.
Title: Sensor size and DoF
Post by: EricV on October 18, 2006, 07:57:47 PM
Quote from: peripatetic,Oct 18 2006, 12:48 PM
I have a particular scene in mind at Kew gardens. It will be a vertical shot of a tree, it has an interesting twisted trunk, and the leaves are turning colour just at the moment. The top will be approximately 30m away from me, and I would like to get the top of the tree sharp and as much of the tree in sharp focus as possible, I would like to know in advance though from what distance I may expect the trunk above me to be sharp for composition purposes and to adjust the angle from which I shoot the tree and tilt the camera.

----------------------------------------------------------------------------------------------

This would be an interesting challenge, which might even lead to something useful, but only if you define the problem unambiguously.  Otherwise everyone is going to interpret your requirements differently and disagree on the conclusion.  At one point, you imply you are shooting from a predetermined location (30m from top of tree, image as part of existing series), but then you imply you are willing to adjust the distance and change the perspective depending on format (angle from which I shoot the tree, printer will print full negative).  You can't do both!  

Let's say you decide the perspective is correct only when shot from a particular location, so that is now fixed.  Hopefully your 150mm lens is sufficient to capture as much of the tree as you want, whether you use a 4x5 back or an 8x10 back or anything else.  Then Howie would argue correctly that the format does not matter.  Of course you cannot get the print you want by printing the full negative in both cases.  Rather, in both cases, you must crop to use the same area of film, containing the part of the tree you are interested in.  Then the final print requires the same enlargement, and I think everyone will agree that the DoF is the same in both cases, because for all intents and purpose they are the same case.  Now that every other variable is fixed, you can consult tables or calculators of depth of field and hyperfocal distance to choose the focus point and f/stop which give you the depth of field you require.

Let's say instead you decide that perspective is unimportant and you want to fill the negative with the tree for both formats.  Since you are stuck with a single lens, you must move closer when using the 8x10, making it harder to achieve the desired depth of focus.  Now your printer can print both negatives full frame, so the 8x10 will be enlarged half as much as the 8x10 to make the final print, making it easier to achieve the desired depth of focus.  You will have to take all of these factors into account to determine the f/stop needed in both cases.  This time you will claim correctly that format does matter, and Howie will complain that you are changing the ground rules.

Let's take one final interesting case.  Suppose you have a 150mm lens for the 4x5 and a 300mm lens for the 8x10.  Now we can make many more vairables constant.  You can shoot from the same location and make essentially the same print in both cases.  This time again format does matter (since magnification is different), but so does lens focal length.  I will leave problem this as an exercise.  At least it is unambiguos and has a single correct answer.
Title: Sensor size and DoF
Post by: BJL on October 18, 2006, 08:35:12 PM
We seem to be back to the old Howard Smith doctrine, which is effectively that the same viewer can arbitrarily choose a different maximum allowable CoC value for judging somethig to be in-focus on different displayed images (prints, on-screen images, etc.) , even when comparing two equal sized prints side-by-side. This renders DOF judgements purely personal and capricious, not subject to any comparisons at all. I can change the DOF on the same print by changing my maximum allowable CoC choice!


I propose this instead: viewer of prints can choose their own personal maximum CoC crietera, and so have different standards for the degree of sharpness needed to deem something "in-focus", but ...

When two images are displayed the same size and viewed from the same distance away, the value of the CoC on the displayed image that divides in-focus from out of focus for any particular viewer is the same for each image.

Thus, if we use different formats and focal lengths and then compare on the basis of equal sized prints presenting equal sized images of the subject, the DOF is the same when the circles of confusion on the prints are all the same size, and if the circles of confusion on the prints are of different sizes, the DOF is less on the print with larger circles of confusion.

This means that the relevant CoC size at the focal plane must be adjusted in inverse proportion to the degree of enlargement needed to get equal sized prints, and so is adjusted in proportion to focal length. Different people might use different values, but the same person will use "camera focal plane" CoC limits that scale this way when the same FOV is photographed with different focal lengths (i.e. with different formats).

Then the rule for camera settings is simple: for images taken from the same position, focused on the same object at the same distance from the camera, and with the same FOV, the DOF is the same if the ratio of the focal lengths is the same as the ratio of the aperture ratios.

Equivalently, the ratio of focal length to aperture ratio is the same in each case.

Equivalently, the DOF is the same if the effective aperture diameter is the same, because
(aperture ratio) = (focal length)/(effective aperture diameter)
so
(effective aperture diameter) = (focal length)/(aperture ratio)
and rule is that DOF is the same when this ratio is the same.


Basic geometrical camera optics shows that in this scenario of equal effective aperture diameter, equal FOV and equal distance from camera to subject (equal focus distance), the circle of confusion at every point of the image produced with the larger focal length is larger than the one at the same part of the smaller focal length image in the same proportion as the focal length is larger. Thus each CoC is larger in the same proportion as the image size is larger, so when the two images are enlarged to give prints of the same size, each circle of confusion on each print is the same size as the CoC at the same point on the other print.

You can't get more equal OOF focus effects and DOF than that, no matter how much personal choice you allow!
Title: Sensor size and DoF
Post by: howiesmith on October 18, 2006, 09:16:05 PM
Quote
We seem to be back to the old Howard Smith doctrine, which is effectively that the same viewer can arbitrarily choose a different maximum allowable CoC value for judging somethig to be in-focus on different displayed images (prints, on-screen images, etc.) , even when comparing two equal sized prints side-by-side. This renders DOF judgements purely personal and capricious, not subject to any comparisons at all. I can change the DOF on the same print by changing my maximum allowable CoC choice!

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I have no idea how you came up with this.  

CoC is personal depending on such factors as eyesight and as basic as what I consider in-focus.  The standard does not change between equal prints.  An 8x10 "print" displayed on a computer monitor is deifferent from an 8x10 chrome on a light table is different from an ink print on an 8x10 piece of paper.  Same image same negative, but not the same "print" and different DoFs for each when viewed by the same viewer.  It simply is easier to see some "prints" than others, even when they are the same size, viewed by the same person from the same distance.

An honest viewer can compare two prints and two really equal prints should appear really equal.  Change one thing and all bets may be off because you just are not looking at equal prints anymore.

Since you seem so sure DoF is format ependant, just point to the optical equations that define DoF and put your finger on "format."  That will satisfy me.  Please jyst show me "format" as an input.  

And don't say it is in the CoC.  CoC depends on the degree of enlargementt of a negative to a print.  If you want to assume DoF is format depenfant by insisting all prints are made from full frame enlergements of the negative (no cropping allowed), then, you are right.  But I claim that is an artificial and totally unnecessary restriction.  You just happen to be enlarging the entire mrgative and the CoC equally.
Title: Sensor size and DoF
Post by: BJL on October 18, 2006, 10:46:32 PM
Quote
Since you seem so sure DoF is format ependant, just point to the optical equations that define DoF and put your finger on "format."  That will satisfy me.  Please jyst show me "format" as an input. 
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Welcome back Howard.

Reread my post and you will not find the words "format dependent", nor any appeal to standard values of the maximum allowable CoC size.

You will find references to images covering the same FOV taken with lenses of different focal lengths. This means that the images formed and recorded at the focal plane of the camera and used to make prints are of different sizes, and that "image size in the focal plane" is perhaps what many people refer to as the format.

If however you wish to avoid that use of the word "format", here is a true statement that should satisfy you. It is based on the basic geometrical optics that determines how large a disk ("circle of confusion") a point is blurred into at the focal plane of the camera.


If one makes equal sized images (prints) of the same scene, with the same FOV and taken from the same position, focused on the same object at the same distance, and using the same effective aperture diameter, the prints will have the same DOF, because indeed at each part of the image, the circle of confusion (the disk into which light from one point is blurred by OOF effects) will be the same size.

Thus, if one image is made using a shorter focal length, the aperture ratio needed to get equal aperture diameter and equal DOF is smaller in proportion to the focal length. Conversely, if the same aperture ratio is used in each case, the shrter focal length gives a smaller effective aperture diameter and so every CoC on the print is smaller in proportion to the focal length, which surely means more DOF.

Forming and recording a smaller image of the same subject (thus using a lens of shorter focal length) and using the same aperture ratio gives more DOF.



The rest of us can interpret "forming and recording a smaller image of the same subject (thus using a lens of shorter focal length)" as "using a smaller format", but you are free to use words otherwise. Using words differently does not change the optical facts stated above.
Title: Sensor size and DoF
Post by: Ray on October 19, 2006, 12:20:30 AM
Quote
Ray,

"The point here is that for DoF purposes it is the format of the image that is actually used in the process of making a print that has a direct bearing on the final DoF perception. "

  I think this is what Howie has been saying all along.  This now becomes clear to me that the word format can mean different things in different contexts or conventions.  If you ask most photographers "What format do you shoot?" they would answer 35 FF, 645 6x7 8x10 etc and the common understanding is that those are the dimensions of the image acquisition area.  In the sentence I quote, you are using the word to refer to the actual image used - therein lies almost all this discussion which seems to revolve around a simple semantic issue here - if you define "format" that way, I suspect Howie would agree...
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Andy,
I think professional and large format photographers have always been used to having a choice of different backs with their cameras. The concept of reducing the format of your camera by reducing the size of the film in the back is only novel to some 35mm users who don't have that choice. I'm sure Howard understands there is no useful distinction to be made between sliding a 6x7cm film holder in the back of a 4x5 camera and using a 4x5" piece of film and cropping it later to 6x7cm. (Except economically, of course. Using a 4x5" piece of film when a 6x7cm piece will do, is very wasteful.)

What is generally referred to as the format of a camera could more precisely be defined as the maximum image size the camera allows you to capture, and that's something that Howard needs to know and does know I'm sure. When Howard picks up his 4x5 camera for a day's shooting, he knows he's got a 4x5 camera and a film holder containg 4x5 film (unless he's totally drunk), and he knows he can not measure the dimensions of any object in his viewfinder which are larger than 5".
Title: Sensor size and DoF
Post by: Craig Arnold on October 19, 2006, 07:41:26 AM
Howie, from your response to my final challenge it is clear to me now that we are both using the same formulae, and as is the way with these things sometimes, have trouble agreeing on how to express them in English. We do not disagree on the mathematics, only on how to say it in words.

The only matter that remains to be resolved (or not) is whether it bothers you that the more common way of expressing these things will cause you to get annoyed at everyone else's stupidity.

At any rate I have been forced to examine the issue more closely, have learned a few things, and enjoyed the exchange.
Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 10:29:08 AM
I apologize for for being angry.  That is not appropriate.  There is a proverb I need to apply:

Proverbs 19, verse 20, "Get all the advice and instruction you can, and be wise the rest of your life."

Let's try this.

The hyperperfocal distance is well accepted to be:

h = f**2/(N*c)

where h is the hyperfoacal distance, f is the lens' focal length, n is the f/stop and c is the circle of confusion diameter on the film.

This equation can be rearrnaged to:

c = f**2/(N*h)

I hope we can all agree that all the terms on the right hand side of the equation are related to the lens and have nothing to do with camera format.  In fact, the lens need not even be on a camera and the c could be measured off a wall, or sidewalk, or whereever the image is focused.  Then, since c is determined be non-format dependant factors, it must also be format independant.

The photographer can change N to adjust c once set up and f and the focus distance is set.  Then DoF can be determined by:

df = d*h/(h-d)

where df is the far limit of DoF, d is the focus distance, and h is the hyperfoacal distance.  I have used a simplified version of the equations for d much much greater than f so d+/-f is essentially d.

The near limit of DoF dn is:

dn = d*h/(h+d)

DoF is the difference between df and dn.

Note: When h=d, dn is h/2 or half the focus distance.  Hence, the notion that DoF is equally divided on either side of the hyperfocal distance.

I hope we can agree that so far camera format (or even a camera) hasn't entered.  But CoC only matters on the print.  Now the c on the format independant film must be put on paper.  Usually enlarged.  So:

CoC on the print = c * enlargement.

The degree of enlargement is also independant of film format.  I think we can all agree that any sized print can be made from any sized negative.  No need to be stuck with 30cm.

Same with viewing distance.  The greater the viewing distance, the greater CoC will need (or can) be to be just seen as a point on the print.  So CoC is variable for viewing ditance and we need not be stuck with 50cm.

Again, I apologize for becoming angry.
Title: Sensor size and DoF
Post by: Ray on October 19, 2006, 10:57:54 AM
Quote
The format size determines how much of the tree is on the negative, and that has nothing to do with DoF.  If I trim an 8x10 neagtive to a 4x5, do I change anything on the negative?  Of course not. [a href=\"index.php?act=findpost&pid=81072\"][{POST_SNAPBACK}][/a]

This is the sort of reasoning that I find bizarre. It's almost like saying, 'if I chop off an arm or a couple of feet, do I change your appearance'.

If you trim the negative before making a print, you have definitely changed the negative. You've removed some of it. If you make 2 prints of equal size, one from the trimmed negative and one from the full negative, you are quite likey going to have prints with different DoFs. Even if you make different size prints by enlarging the elements that are common to both images by the same degree, you can still have the perception of different DoFs, even though the DoF of the elements that are common to both prints is the same.

For example, if the full shot of the tree contains some buttresses (above ground roots) which are out-of-focus because I made a mistake with my DoF calculations or adjustments, I might decide to crop that part of the negative. (We all know that stuff in the foreground that's OoF is often not acceptable.) By cropping away the 'mistakes', I create a different composition, perhaps as good, perhaps not as good, but one which now has the correct or acceptable DoF.

On the other hand, the reason for cropping might not be to correct a mistake. If I have a fixed format camera, a prime lens with a fixed aperture and not much room to change my distance to the tree, I might well make the choice to sacrifice part of the negative, or use a smaller format (a film holder with a 6x7cm piece of film, if I have one).

To put it another way, from the perspective of the viewer of a print, DoF is correct, satisfacory, acceptable or not, as the case may be, only in relation to the format used. Changing format does not change the hypothetical range of distances where elements will be OoF by the specified degree in accordance with the rules of DoF but it does make the settings of f stop, distance to subject and choice of focal length either appropriate or not appropriate for the composition.

It's my view that the confusion in this thread results from a failure to distinguish clearly between the perception of DoF on the finished print and the hypothetical, imaginary DoF described by the mathematics, which of course must remain imaginary. I don't believe I have ever captured infinity on sensor or film, yet the DoF tables will often tell me that everything is sharp from x feet to infinity.
Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 11:50:05 AM
Quote
I apologize for for being angry.  That is not appropriate.  There is a proverb I need to apply:

Proverbs 19, verse 20, "Get all the advice and instruction you can, and be wise the rest of your life."

Let's try this.

The hyperperfocal distance is well accepted to be:

h = f**2/(N*c)

where h is the hyperfoacal distance, f is the lens' focal length, n is the f/stop and c is the circle of confusion diameter on the film.

This equation can be rearrnaged to:

c = f**2/(N*h)

I hope we can all agree that all the terms on the right hand side of the equation are related to the lens and have nothing to do with camera format.  In fact, the lens need not even be on a camera and the c could be measured off a wall, or sidewalk, or whereever the image is focused.  Then, since c is determined be non-format dependant factors, it must also be format independant.

The photographer can change N to adjust c once set up and f and the focus distance is set.  Then DoF can be determined by:

df = d*h/(h-d)

where df is the far limit of DoF, d is the focus distance, and h is the hyperfoacal distance.  I have used a simplified version of the equations for d much much greater than f so d+/-f is essentially d.

The near limit of DoF dn is:

dn = d*h/(h+d)

DoF is the difference between df and dn.

Note: When h=d, dn is h/2 or half the focus distance.  Hence, the notion that DoF is equally divided on either side of the hyperfocal distance.

I hope we can agree that so far camera format (or even a camera) hasn't entered.  But CoC only matters on the print.  Now the c on the format independant film must be put on paper.  Usually enlarged.  So:

CoC on the print = c * enlargement.

The degree of enlargement is also independant of film format.  I think we can all agree that any sized print can be made from any sized negative.  No need to be stuck with 30cm.

Same with viewing distance.  The greater the viewing distance, the greater CoC will need (or can) be to be just seen as a point on the print.  So CoC is variable for viewing ditance and we need not be stuck with 50cm.

Again, I apologize for becoming angry.
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Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 11:56:24 AM
Quote
I apologize for for being angry.  That is not appropriate.  There is a proverb I need to apply:

Proverbs 19, verse 20, "Get all the advice and instruction you can, and be wise the rest of your life."

Let's try this.

The hyperperfocal distance is well accepted to be:

h = f**2/(N*c)

where h is the hyperfoacal distance, f is the lens' focal length, n is the f/stop and c is the circle of confusion diameter on the film.

This equation can be rearrnaged to:

c = f**2/(N*h)

I hope we can all agree that all the terms on the right hand side of the equation are related to the lens and have nothing to do with camera format.  In fact, the lens need not even be on a camera and the c could be measured off a wall, or sidewalk, or whereever the image is focused.  Then, since c is determined be non-format dependant factors, it must also be format independant.

The photographer can change N to adjust c once set up and f and the focus distance is set.  Then DoF can be determined by:

df = d*h/(h-d)

where df is the far limit of DoF, d is the focus distance, and h is the hyperfoacal distance.  I have used a simplified version of the equations for d much much greater than f so d+/-f is essentially d.

The near limit of DoF dn is:

dn = d*h/(h+d)

DoF is the difference between df and dn.

Note: When h=d, dn is h/2 or half the focus distance.  Hence, the notion that DoF is equally divided on either side of the hyperfocal distance.

I hope we can agree that so far camera format (or even a camera) hasn't entered.  But CoC only matters on the print.  Now the c on the format independant film must be put on paper.  Usually enlarged.  So:

CoC on the print = c * enlargement.

The degree of enlargement is also independant of film format.  I think we can all agree that any sized print can be made from any sized negative.  No need to be stuck with 30cm.

Same with viewing distance.  The greater the viewing distance, the greater CoC will need (or can) be to be just seen as a point on the print.  So CoC is variable for viewing ditance and we need not be stuck with 50cm.

Again, I apologize for becoming angry.
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Forgot to add, if you know the degree of enlargement is going to be large (perhaps due to cropping), you might want to adjust your personally selected CoC accordingly.  These factors, beins subjective, are not cast in concrete.

Image planning seldom takes into account I am making a mistake in plnning.  If I knew that, I would just correct the plan.  There may be times that the phyical reality of the subject's dimensions will not be the same as the plan you made (the tree grew).  Then you may need to replan, not just change the degree of enlargment and crop.
Title: Sensor size and DoF
Post by: Eric Myrvaagnes on October 19, 2006, 12:23:31 PM
I have been lurking this thread from the beginning, wondering how much anyone could possibly have to say about DOF in this day and age.

A friend of mine just getting into photography some 40 years ago felt quite guilty about having a hard time understanding explanations of DOF that she had heard. I think if she read this entire, massive thread today, she might feel better. And I think Proust would be proud of all of you.

For a while you all seemed a bit like the blind men describing an elephant, but eventually -- to me, anyway -- the views on both sides became clearer. At this point I'm tempted to say "you're both right."

Like the farmer listening to a political debate: After each politician spoke, the farmer turned to his neighbor and said, "By golly, he's right!" After the second such comment, the neighbor said, "But they can't both be right, can they?" To which the farmer replied, "By golly, you're right, too!"

My apologies to any farmers on the forum. In a story like this, you've got to pick on some innocent party.

Eric
Title: Sensor size and DoF
Post by: EricV on October 19, 2006, 12:31:32 PM
Quote from: howiesmith,Oct 19 2006, 07:29 AM
c = f**2/(N*h)
I hope we can all agree that all the terms on the right hand side of the equation are related to the lens and have nothing to do with camera format.  
--------------------------------------------------------------------------------------------

I believe most photographers would NOT agree that this equation has nothing to do with camera format, because most photographers consider the lens focal length and the camera format to be closely related.

If I decide to take a picture of a certain object from a certain location and I have a camera of a certain format, I will certainly try to choose the lens focal length to fill the frame.  If I switch to a different format, this means choosing a lens of a different focal length.  Yes, I could use a short lens on a large format and crop the image, but I will try to avoid doing this if possible.  I would prefer to use a longer lens to fill the frame and then stop the lens down more to get the required DoF.

I know you disagree with this conceptual linking of format and lens, but this sort of disagreement is just semantics, with no real content, and it is getting tiresome.  Can we agree that DoF is the same if I use the same lens and crop the larger format, but that DoF is not the same if I use a different lens to give the same field of view on a different format?  If we agree on this, then it does not really matter if we disagree on vague statements like "format does not affect DoF".
Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 12:59:25 PM
Quote from: EricV,Oct 19 2006, 10:31 AM
Quote from: howiesmith,Oct 19 2006, 07:29 AM
c = f**2/(N*h)
I hope we can all agree that all the terms on the right hand side of the equation are related to the lens and have nothing to do with camera format.  
--------------------------------------------------------------------------------------------

I believe most photographers would NOT agree that this equation has nothing to do with camera format, because most photographers consider the lens focal length and the camera format to be closely related.

If I decide to take a picture of a certain object from a certain location and I have a camera of a certain format, I will certainly try to choose the lens focal length to fill the frame.  If I switch to a different format, this means choosing a lens of a different focal length.  Yes, I could use a short lens on a large format and crop the image, but I will try to avoid doing this if possible.  I would prefer to use a longer lens to fill the frame and then stop the lens down more to get the required DoF.

I know you disagree with this conceptual linking of format and lens, but this sort of disagreement is just semantics, with no real content, and it is getting tiresome.  Can we agree that DoF is the same if I use the same lens and crop the larger format, but that DoF is not the same if I use a different lens to give the same field of view on a different format?  If we agree on this, then it does not really matter if we disagree on vague statements like "format does not affect DoF".
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Belive what you will.  I certainly can't change that.  Only you can do that.

I think the equations are for lenses, not just camera lenses.  I do agree that most photographers tend to think in terms of the camera(s) and lens(es) they have.  I just don't see the need for one set of rules for each camera and lens.

If lens focal length is linked to format, how is it I can put a 150mm lens on APS, 35mm, 645, 6x6, 4x5 camera?  And they all produce the same focused image, just more or less of the world?  I even once put my 6x6 50mm lens on my 4x5 camera.  Same lens, different format camera and all the "laws of optics" went right along.  There were other problems, but DoF was not one of them.

If your goal is to fill the frame, then, yes, I agree it is a good idea to know the frame size and have a bunch of focal lengths on hand.  My goal is to produce an image for a print.  This only requires the format to be 1) big enough to get the photo I want on it, and 2) I have a format that will do that.  These are not always possibel for me.  I can't photograph everything.  But that is a hardware problrm, not a DoF problem.  Sometime it can be equipmant if I need f/90 to get the DoF I want (planning).  That is a lens problem and a plan problem, not a DoF problem.  Not every plan is actually workable with the equipmant I have.  I don't see any use in planning a print I can't take.  Even if you can.  But then scrambling around, zooming in and out, set the f/stop to max sharpness, let my camera focus (some how), set an exposure somehow, and then bracket like crazy isn't my idea of a solution or a plan either.

If your goal is to amke only 30cm prints from full frame negatives to be viewed by an average person under average conditions at 50cm, then maybe on-line calcualtors are just what the doctor ordered, for you.  Please just don't say they are for everyone and every combination of conditions
Title: Sensor size and DoF
Post by: Ray on October 19, 2006, 01:14:02 PM
Quote
Can we agree that DoF is the same if I use the same lens and crop the larger format, but that DoF is not the same if I use a different lens to give the same field of view on a different format?  If we agree on this, then it does not really matter if we disagree on vague statements like "format does not affect DoF".
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Well, I certainly can't agree with that. It's quite possible to get the same DoF when using a different lens on a different format shooting from the same distance, if you make an f stop adjustment.

An example would be the 20D with a 50mm lens at f5.6 and the 5D with 80mm lens at f9 (approximately). Because of the different pixel densities (and pixel count) of the 2 formats the precise difference in f stops would probably be in fractions not available in the camera settings. In this example, multiplying the 50mm focal length, and whatever f stop used, by 1.6 gives a good approximation.

I can't agree that DoF on a print is the same if I use the same lens (and presumably you mean same f stop and distance to subject) and crop the image, because the print will be different. As I've already tried to explain, the print from the uncropped image (negative) might contain lots of fuzzy elements giving the impression of a shallow DoF. The print from the cropped image might be tack sharp from corner to corner because all the fuzzy elements have been cropped out.

The only thing about DoF that format doesn't and cannot change is the mathematical description of it which is infinitely bigger than any physical format. It stretches all the way to infinity   .
Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 01:37:46 PM
Wiki says:

"... separation of variables is any of several methods for solving ordinary and partial differential equations, in which algebra allows one to re-write an equation so that each of two variables occurs on a different side of the equation."

Format on one side, DoF on the othr.  Change format and only format and watch the change in DoF.  Only problems here, some don't believe the math, and format doesn't appear opposite DoF - not related.  Sure, you can replace c woth CoC (CoC=c8enlergement) and you still don't get format.

One might also look at if I change DoF (and only DoF), how does format change?  Doesn't, not related.
Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 01:45:44 PM
Quote from: EricV,Oct 19 2006, 10:31 AM
Quote from: howiesmith,Oct 19 2006, 07:29 AM
c = f**2/(N*h)
I hope we can all agree that all the terms on the right hand side of the equation are related to the lens and have nothing to do with camera format.  
--------------------------------------------------------------------------------------------

I believe most photographers would NOT agree that this equation has nothing to do with camera format, because most photographers consider the lens focal length and the camera format to be closely related.

[a href=\"index.php?act=findpost&pid=81176\"][{POST_SNAPBACK}][/a]

EricV, I think most people used to believe the earth was flat, the earth was the center of the universe, and Zeus was sitting on top of Olympus as king of the gods.

I don't think most people believe all that now, nor do I belive there was a major restructuring done to make those things untrue.  They just weren't true to begin with, regardless of how many and who believed it.  But I also belive the changes in beliefs were hard to make.  Poor Gallileo.
Title: Sensor size and DoF
Post by: EricV on October 19, 2006, 02:08:19 PM
Good comments, EricM.  This discussion is getting ridiculous.  Everyone agrees on all the equations, meaning we really do understand DoF and can use it properly in our photography.  The only disagreements concern the language used to discuss it.

If I work by first selecting a lens, then adjusting the f/stop to give the DoF I desire for the final print I intend to make (using the equations we all agree on), then I am entitled to say "format does not affect DoF" (as long as it covers the field of view I am interested in and I don't mind cropping).

If I work by first selecting a film format, then choosing a lens to capture the field of view I am interested in, then adjusting the f/stop to give the DoF I desire for the final print I intend to make (using the equations we all agree on), then I am entitled to say "format affects DoF".  Or maybe I have to say "format determines lens focal length which affects lens f/stop" to avoid offending anyone.

In both cases, if we all intend to make the same print, with the same field of view and the same depth of field, we will all end up choosing the same f/stop if we happen to use the same lens (independent of format), and we all agree we will have to use different f/stops to achieve similar DoF if we happen to select different lenses (driven by our choice of format).

Maybe, just maybe, we can all agree on this?  In any case, I don't think there is really much more to discuss.
Title: Sensor size and DoF
Post by: howiesmith on October 19, 2006, 02:33:54 PM
Quote
If I work by first selecting a film format, then choosing a lens to capture the field of view I am interested in, then adjusting the f/stop to give the DoF I desire for the final print I intend to make (using the equations we all agree on), then I am entitled to say "format affects DoF".  Or maybe I have to say "format determines lens focal length which affects lens f/stop" to avoid offending anyone.

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Oh, I would agree with this.  If I assume DoF is format dependant, it is far easier to prove I am right.  And you are right within your assumptions

Where you may get in difficulty, is changing those assumption as you go,  Like making less than a full frame enlargement.  I frequently (maybe always) figure on using only about 90% of the available format to give myself some wiggle room later.  This works but I have never been able to get that part of the image that falls just outside the film.

And working my way, I can also plan a full frame enlargement into a 30cm print to be viewed from 50cm by the average person

I find it easy to go from the general to the particular case, but not always possible to go the other way.
Title: Sensor size and DoF
Post by: Ray on October 19, 2006, 07:32:20 PM
Quote
If I work by first selecting a lens, then adjusting the f/stop to give the DoF I desire for the final print I intend to make (using the equations we all agree on), then I am entitled to say "format does not affect DoF" (as long as it covers the field of view I am interested in and I don't mind cropping).

Maybe part of the confusion is due to the different work flow and procedures some people follow in making a picture. Personally speaking, I always look for the picture first. All pictures are bound within a format. I then select an approprite lens and distance to subject and make a crop. I always make a crop. It is impossible not to do so. A picture is by definiotion a crop. A cropless, formatless picture is an impossibility.

Sometimes it is impossible to take the picture because (1) the camera's format is too small, (2) I haven't got a lens wide enough, (3) I can't get far enough away from, or sufficiently close to, the subject.

The DoF mathematical formulas encompass the total range of possible (as well as  impossible) pictures that can be made when shooting from a particular position. The number is infinite. You choose one and one only at a particular moment at a particular position when taking a picture. All characteristics of that one chosen picture are affected by format, including DoF. A picture cannot exist outside of a format. It's the choice of format that nails the selection of DoF since the scene in front of you, from the perspective of your lens (or position of your lens), will contain the whole range of DoF possibilities. Within that range, there are some pictures that are completley OOF, some that are completely sharp from corner to corner and some that are a mixture.

The making of a picture involves a subjective choice. The DoF formulas of course do not know the size of the image circle of your lens, or the sharpness of your lens, or the pixel density of your sensor, or anything about your choices. The DoF formulas are merely presenting the range of choices. If that's all Howards is saying, then of course I agree with him. I actually thought he was talking about making pictures and how the perception of DoF on the print  (or negative for that matter) is affected by format.
Title: Sensor size and DoF
Post by: BJL on October 20, 2006, 10:55:09 AM
Quote
Format on one side, DoF on the othr.  Change format and only format and watch the change in DoF.  [a href=\"index.php?act=findpost&pid=81188\"][{POST_SNAPBACK}][/a]
Unfortunately, it is usually impossible in practice to change to a smaller format without changing one of the
- field of view
- focal length
that one got with one's usual procedure with a larger format.

For example, if I have photographed a certain scene with my 35mm film camera, choosing a focal length to more or less fill the frame with the desired subject matter, then using the same focal length with my 4/3 format DSLR will give a far narrower FOV, recording only about one quarter of the original subject area. Thus,

the smaller format forces me to use a shorter focal length in order to get the same composition as I previously got with the larger format.

There is the alternative of going back to the larger format and matching the FOV and DOF of the smaller format by using this same new shorter focal length and then cropping heavily, but that is not the situation that I or most photographers are interested in when we ask how DOF will be affected by changing to a smaller format.


P. S. If one wants formulas, one is
2*tan(AFOV/2) = (format diagonal length)/(focal length)
where AFOV is angular field of view of the image recorded by the sensor/film, measured on the diagonal.
(There are similar ones got by replacing "diagonal" by "horizontal" or "vertical" thoughout.)

Format dimensions do appear in such formulas.
Title: Sensor size and DoF
Post by: howiesmith on October 20, 2006, 11:31:00 AM
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the smaller format forces me to use a shorter focal length in order to get the same composition as I previously got with the larger format.

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I don't disagree with that.  But I can reduce the focal length without changing format to get the same image, just more of it.  One more time (and last), if you assume format is a factor by insisting that the film not be cropped and the entire photo you had in mind (planned) fits on the film, then it is easy to prove a format dependance.

The smaller format you were speaking of seems to fail one condition for planning an image, namely the format selected out of necessity (what I have) be big enght to get the planned photo on.  A 6x6 with a 159mm lens is plenty large enogh to get all the image on the film that an APS camera with a 150mm lens is.  But if I make my plans with a 6x6 and then decide to use an APS, the 150mm lens might not work.  I may have to switch my plan to a shorter focal length, or back up.  BUT, I have changed my plan, and if I change the ground rules, I may have to change a lot of factors.  That doesn't make DoF format dependant.

If I change formats, and then lens focal length (or focus distance) to get the photo back into the format, DoF calcs need to be redone, nnot because of the format change, but because I changed focal length (or distance).

The actual scene being photographed may not meet all the plans.  When I go to the Grand Canyon, I don't plan a full width image of the Canyon.  I don't have a lens that wide.  I could plan that shot assuming a fisheye lens, but that would be a waste of time because I don't have one.  Likewise, a plan may require f/1.4 to get the narrow DoF I would like.  That is interesting, but since I don't have any f/1.4 lenses, not an implementable plan, as is.

If I had to change formats to get all of the Grand Canyon in, would the Grand Canyon become format dependant?  If I don't have the equipment or it isn't even available, does that make the Grand Canyon unphotograhable?  No, just certain planned images.  I can make a plan that would reuire a 15mm lens at f/1.4 on an 8x10 camera.  Doesn't mean I can do it.  (And I chose an 8x10 camera not for DoF, but as big enough to get on film.  That is still an inportant and necessary factor.  But if I had that lens and an 11x14 camera, I could still get the same shot with the very same DoF as planned for the 8x10 camera.)

You need to be very careful when changing variables that you do not couple them with othr variables, and then attribute all or part of the change to the origianl variable.  That is, if you change format and lens focal length and get a different DoF, don't attribute that change to format.
Title: Sensor size and DoF
Post by: BJL on October 20, 2006, 12:07:13 PM
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I can reduce the focal length without changing format to get the same image, just more of it.
That is not the same image: it is a different image, covering a larger FOV, that contains the prevoius smaller format image as only a part of the whole. A "head shot"  is not the same image as a full length portrait.

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One more time (and last), if you assume format is a factor by insisting that the film not be cropped and the entire photo you had in mind (planned) fits on the film, then it is easy to prove a format dependance.[a href=\"index.php?act=findpost&pid=81354\"][{POST_SNAPBACK}][/a]
And one more time, I am making no assumptions: I am stating explicitly that I am interested in what happens when I photograph a given scene using most or all of the area covered by the respective sensor/film formats. This is no more an "assumption" than are other comparison conditions, like focusing on the same subject from the same distance.


You are a simply stating the true but rather uninteresting fact that a camera with a larger sensor or film format can be used to more or less mimic the results of a smaller format, by suitable cropping.

Indeed one could use a  5D with 24x36mm format sensor and crop to the roughly 13.1x17.5mm format of the E-1, and have a rather heavy, expensive way to get 3.4MP images in the 13.1x17.5mm format of 4/3".

Yes, that cropped image has format 13.1x17.5mm, not 24x36mm, even if the camera is capable of producing images of larger formats when used with other cropping choices.

Or one could us the D2X with its 15.7x23.5mm sensor in high speed mode, which records an image from about a 12x18mm portio of the sensor: what format is that image? I would say it is 12x18mm, and I would say the same if one used normal mode, recorded a 15.7x23.5mm image and the cropped it to the same framing given by high speed mode.


How about we describe the format of an image as the height and width of the image formed in the focal plane of the camera and used to make the final print. Then when we crop, the cropped image is has a new, smaller format. Then it is clear that when photographing the same scene (FOV, subject distance, focus distance) using an image of smaller format, the aperture ratio must be reduced in proportion to the linear size of the image format in order to get the same DOF, while using the same f-stop with that smaller image format gives more DOF.
Title: Sensor size and DoF
Post by: howiesmith on October 20, 2006, 12:30:15 PM
Quote
And one more time, I am making no assumptions: I am stating explicitly that I am interested in what happens when I photograph a given scene using most or all of the area covered by the respective sensor/film formats. This is no more an "assumption" than are other comparison conditions, like focusing on the same subject from the same distance.

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My mistake.  I was interchanging a problem given (clearly stating the format selected is completely filled with only the photo I want) with an assumption.  In fact, that mwakes it even easier.  No need to assume DoF is format dependant, I am told it is.  "It is because I said so."  That makes even possible to prove pigs can fly.  "Given pigs can fly at 100 mph, how long will it take a pig to fly 200 miles?"

Wrong of me to do that.  However, I don't think it changes much of the discussion.  Just the difference in what is an assumption and what is a given.

The I guess that lens makers are not "assuming" a value of CoC.  It is just a "given" not stated very clearly.

Now we are really splittint hairs to prove DoF is format dependant.
Title: Sensor size and DoF
Post by: BJL on October 20, 2006, 05:19:15 PM
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No need to assume DoF is format dependant, I am told it is.  "It is because I said so."  That makes even possible to prove pigs can fly.  "Given pigs can fly at 100 mph, how long will it take a pig to fly 200 miles?"
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Howard, your analogy is ridiculously irrelevant. It turns on asserting a falsehood. (At least, I take it that you intend it to be clearly false that pigs can fly at 100 mph. If instead pigs can do that, there is no point to your analogy, and the answer is two hours, as surely as my answer is that DOF at equal aperture ratio increases.)

Where is the false assertion or assumption in my argument?

All I see in my argument is th statement that I am considering the quite real case of photographs taken with the larger format camera composed to fill most or all of the frame with the desired subject matter, so that little or no cropping is needed. This happens far more often than pigs fly at 100mph without mechanical assistance.
Title: Sensor size and DoF
Post by: howiesmith on October 20, 2006, 05:28:34 PM
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Howard, your analogy is ridiculously irrelevant. It turns on asserting a falsehood. (At least, I take it that you intend it to be clearly false that pigs can fly at 100 mph. If instead pigs can do that, there is no point to your analogy, and the answer is two hours, as surely as my answer is that DOF at equal aperture ratio increases.)

Where is the false assertion or assumption in my argument?

All I see in my argument is th statement that I am considering the quite real case of photographs taken with the larger format camera composed to fill most or all of the frame with the desired subject matter, so that little or no cropping is needed. This happens far more often than pigs fly at 100mph without mechanical assistance.
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Title: Sensor size and DoF
Post by: howiesmith on October 20, 2006, 05:50:16 PM
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All I see in my argument is th statement that I am considering the quite real case of photographs taken with the larger format camera composed to fill most or all of the frame with the desired subject matter, so that little or no cropping is needed. This happens far more often than pigs fly at 100mph without mechanical assistance.

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And all I have been trying to say is it is an unnecessary given or assumption that the photo fill the frame, even if that is the way most photographers work.  All that you are saying by making that requirement is the film format and the image of the CoC are enlarged the same (true) and that the full photo will now fill the print (not necessarily true).

And I think you are saying the full format will never be larger than the piece of paper it is being printed on (clearly wrong).  I can certainly try to ptint a 2x enlargement of a 4x5 format film (8x10) on a 5x7 piece of paper.  I won't all of the stuff on the piece of film, but what I do get will be exactly the same as if I used an 8x10 piece of paper and the DoF will be exactly (not nearly) the same.  SAme as if I printed the film on an 8x10 piece of paper and used my paper trimmer to make a 5x7.  What is left is exactly the same as it was , but cause it is what it was before.  That is very simple and should be easy to follow.  

I do not have to make a full frame enlargement.  

And yes, it is possible that the full photo intended is too big to fit on the full frame.  Try to tell me you have never wished you had a little more of that scene.   But that does not keep me from making the same print I had planned from the captured part with the same DoF.

My example about the pigs was intended only as an example.  I do not really think pigs fly.  But if the given for the problem is pigs do fly, then the aswer will reflect that pigs do fly.  You said it would take the pig 2 hours to fly 200 miles.  It requires no trueth or faith about flying pigs.
Title: Sensor size and DoF
Post by: howiesmith on October 20, 2006, 06:29:31 PM
BJL,

I have a negative with a man's image on it.  The man's image is 1" high.  

1)  How many times must I enlarge the negative to make a print of the man where the man is 2" high?

2)  What format is the film?

Of course the answer to 1) is 2x.

The answer to 2) has to be 1xsomething because all images fill the frame.  I say wrong.  Mightn't it be possible to have a 1" high image of a man on a 645?  Or a 6x6?  Or a 4x5?  I think the only requirement is the film be at least 1" high (not only 1" high). And would the 2" high print of the man look exactly the same regardless of whether you printed a 645, 6x6 or 4x5 or even a 1xsomething?
Title: Sensor size and DoF
Post by: Ray on October 20, 2006, 07:28:30 PM
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I say wrong.  Mightn't it be possible to have a 1" high image of a man on a 645?  Or a 6x6?  Or a 4x5?  I think the only requirement is the film be at least 1" high (not only 1" high). And would the 2" high print of the man look exactly the same regardless of whether you printed a 645, 6x6 or 4x5 or even a 1xsomething?
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I thought we'd got beyond the notion that format only refers to camera format. Cropping the image bound by the camera's format is common practice even amongst users of small cameras. To make a picture, you have to make a crop of the sene in front of you. The maximum size of that crop, with a given focal length, is the format of the camera.

It's perfectly true that a 1" high figure enlarged 2x will be 2" whatever the original format of the camera, provided the format is at least 1" high or wide. But doesn't DoF refer to the relative sharpness of different elements within a composition? Are we talking here about 'cut-outs' with no background?
Title: Sensor size and DoF
Post by: Ray on October 21, 2006, 11:13:56 PM
To expand upon my previous post, the problem with many of Howard's examples attempting to support his assertion that format is unattached to DoF, is that essentially they are all saying that A=A, or if A=B then B=A.

There's no great insight here. If I cut a print in half, then of course that half is the same as the half before it was cut, but that half is clearly not the same as the uncut whole.

As BJL has mentioned, if we are comparing the perception of DoF on 2 prints, we should keep the prints which are under comparison the same size to reduce the number of variables. We should also not vary the quality of the viewer's eyesight or the viewing distance to the 2 prints.

Howard has brought up these subjective variables time and again. They are really just a smokescreen.

I shall now attempt a definitive proof that changing format can directly affect the perception of DoF on a print when comparing equal size prints of the same subject and same FoV.

I'll use a range of f stops that are more familiar to users of 35mm, rather than 4x5 format.

I'll use Howard's example of a full height figure. Here's the scene; Howard standing in front of a huge tree with spreading canopy. The background stretches to a theoretical infinity, mountains on the horizon, other trees and tall grass in between. Howard is very close to the tree trunk, in fact leaning back on it. I have a prime lens which, from a convenient position, takes in a good portion of the scene in front of me. No need to crop smaller than the camera's format. I calculate that f4 is sufficient to get all of Howard sharp on a good size print, from the tip of his nose to the patch of ground he is standing on, to the hair at the back of his head, to the tree trunk he is leaning against. For good measure, I use f5.6, just in case I later want to make a large print of a crop.

However, I'm a bit concerned about the close foliage at the top of the frame, which at f5.6 is definitely going to be out-of-focus. The mountains and other trees in the background will also be OoF, so I take another shot at f13 to get everything looking sharp, even in a large print.

I make a 16x24 print of each shot. The one at f5.6, as expected, has a shallow DoF, but the one at f13 appears equally sharp from corner to corner. (I'm actually using the Canon TS-E 45mm which has a larger than usual image circle and produces a reasonably good result from corner to corner.)

I show Howard the 2 prints and he agrees (as anyone would) that the f13 shot has much greater DoF, but Howard doesn't like all the distracting foliage and asks me to make a crop of just him against the tree trunk, which I do, cropping to a panoramic aspect ratio of 3:1, vertically orientated.

I crop both images. I'm not sure why I bothered cropping both images, but Howard told me that cropping or changing format does not change DoF, so I expected one print might still exhibit greater DoF than the other. (Not really   ).

Lo and behold! I find that both prints, still both equal in size, now have identical DoF.

By changing just one variable, the format, I have changed the DoF on the final result, the print. Of course, by cropping I have changed the composition and FoV, but that's what cropping does when you use the same lens. It's unavoidable. The cropped print is also smaller than the uncropped print, but again, that's what cropping does.

Perhaps we could summarise this principle as follows.

If 2 prints of the same scene have equal DoF, cropping to a different format will also result in 2 prints with the same DoF. However, if 2 prints have an unequal DoF, then cropping (ie. changing format) can change that inequality.
Title: Sensor size and DoF
Post by: BJL on October 23, 2006, 11:52:25 AM
Howard,

   we agree that if you crop or enlarge suitably (in this case to what I am inclined to call a 1" high image format), you can get the same DOF with the same aperture ratio from different size pieces of film or sensors.

My statement is about a different situation, which I believe is far more common:
what happens when one roughly fills the frame with the same subject in each case (and then compare prints on which the image of the subject is the same size, viewed from the same distance). This leads to the use of different focal lengths and to different DOF with equal aperture ratio.

Or as I like saying: once you specify
- aperture size (effective aperture diameter, which is focal length divided by aperture ratio)
- focus distance
- FOV on the final print (after possible cropping)
- print size, and
- print viewing distance
then DOF is not effected by choices like format or focal length.
(But if focal length varies, aperture ratio is varied in proportion, to keep aperture size the same.)


P. S. Format does have some effect in your case, since any format with a frame less that 1" high forces the use of a shorter focal length in order to fit the man's image into the format of the frame. You come close to assuming or declaring a falsehood when you talk of having an image 1" high and yet are trying to argue that this is a format independent situation.


 and thus more DOF at equal apertuer ratio.
Title: Sensor size and DoF
Post by: BJL on October 23, 2006, 12:02:35 PM
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And all I have been trying to say is it is an unnecessary given or assumption that the photo fill the frame, even if that is the way most photographers work.
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And all I am saying is what happens in that common and interesting situation, without denying for a moment that other approaches are sometime taken.

Why does this bother you, so long as what I say about that situation is true, and I make it explicit that I am referring to that situation, with no hidden assumptions?


I in turn agree that if one uses films of various different formats but then view only the part of the image recorded on a certain sized part of the film (say 1" high or 24, by 36mm), then the DOF is not affected by the format (height and width) of the whole piece of film, only by the format (height and width) of the piece of film on which the viewed image is recorded.