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Author Topic: Newbe question 35mm vs MF and dof  (Read 41802 times)

Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #80 on: January 03, 2011, 05:09:47 pm »

David,

The definition of DOF has an assumed COC as parameter. Without the COC parameter, DOF cannot be calculated. Use a different COC, and you'll get a different DOF.

Cheers,
Bart

David, what part of this do you not agree with? Seems pretty clear cut and easy to understand to me.

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Re: Newbe question 35mm vs MF and dof
« Reply #81 on: January 03, 2011, 05:58:45 pm »

David,

You are confusing resolution in the imaging plane with DOF. The definition of DOF has an assumed COC as parameter. Without the COC parameter, DOF cannot be calculated. Use a different COC, and you'll get a different DOF.

Cheers,
Bart

David seems to be under the impression that the CoC used to calculate the DoF is a "fact" ...

http://en.wikipedia.org/wiki/Circle_of_confusion

In photography, the circle of confusion diameter limit (“CoC”) for the final image is often defined as the largest blur spot that will still be perceived by the human eye as a point.
With this definition, the CoC in the original image (the image on the film or electronic sensor) depends on three factors:

1) Visual acuity. For most people, the closest comfortable viewing distance, termed the near distance for distinct vision (Ray 2000, 52), is approximately 25 cm. At this distance, a person with good vision can usually distinguish an image resolution of 5 line pairs per millimeter (lp/mm), equivalent to a CoC of 0.2 mm in the final image.

2) Viewing conditions. If the final image is viewed at approximately 25 cm, a final-image CoC of 0.2 mm often is appropriate. A comfortable viewing distance is also one at which the angle of view is approximately 60° (Ray 2000, 52); at a distance of 25 cm, this corresponds to about 30 cm, approximately the diagonal of an 8″×10″ image. It often may be reasonable to assume that, for whole-image viewing, a final image larger than 8″×10″ will be viewed at a distance correspondingly greater than 25 cm, and for which a larger CoC may be acceptable; the original-image CoC is then the same as that determined from the standard final-image size and viewing distance. But if the larger final image will be viewed at the normal distance of 25 cm, a smaller original-image CoC will be needed to provide acceptable sharpness.

3) Enlargement from the original image to the final image. If there is no enlargement (e.g., a contact print of an 8×10 original image), the CoC for the original image is the same as that in the final image. But if, for example, the long dimension of a 35 mm original image is enlarged to 25 cm (10 inches), the enlargement is approximately 7×, and the CoC for the original image is 0.2 mm / 7, or 0.029 m
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #82 on: January 03, 2011, 06:04:15 pm »

Nick, your statement confirms exactly what I have been saying all along.  DOF is defined in terms of real depth.  It is not something that is defined by any illusion of depth.  In other words, the equations of optics that define DOF have nothing to do with the illusion of depth as it would appear on the 2D captured medium.

So, I think you actually must agree with me on my original claim in this thread:  The DOF cannot depend on print size, since real depth (DOF) and illusion of depth (print) are two entirely different things.  

Do you now see your misunderstanding?  The illusion of depth in the print depends on the DOF of the captured image, but the opposite is not true.


I do see what you are getting at. However...

Firstly:

"The illusion of depth in the print depends on the DOF of the captured image, but the opposite is not true." Show me why this is not true. The point that you are missing is that the final print (enlargement/magnification) defines what the DOF of the capture actually means. This is the whole point. if you disagree with this maybe you'd like to share with us what CoCs really mean.

Secondly:

"The DOF cannot depend on print size, since real depth (DOF) and illusion of depth (print) are two entirely different things.  "

There is no such thing as 'real' depth since this term has no meaning unless you define a CoC, which is itself determined by the magnification of the image into a print, monitor or whatever. This is the other point you are failing to understand. There is absolutely no 'intrinsic' depth of field in an image since DOF can only be meaningfully defined if you specify an appropraite CoC. This has been pointed out repeatedly.

The only case in which your position is in fact correct is when you use the old standard for DOF calculations based on a 10x8 print at 25cm viewing distance and based on an image CoC of 0.029mm. This CoC is calculated based on the original being enlarged by a fact of about 7 so the CoC at print size becomes 0.2mm which is considered the minimum resolvable detail at the 25cm viewing distance. Look up these figures, plug them into a DOF calculator, and you'll see it's pretty well understood by (almost) everyone.

Calculating your DOF from this CoC (0.029mm) will give you meaningful DOF only as long as you only make a 10x8 print. If you make a 20x16 print your CoC calculation needs to be based on a CoC of 0.015mm so the roughly 14 times enlargement still results in the 0.2mm print CoC. Therefore the DOF can be said to be halved in the bigger print.

Lastly, and how you can dispute this I cannot imagine, have a close look at the attached screenshots from a DOF calculator. Half the CoC results in half the DOF.

QED.

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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #83 on: January 03, 2011, 06:19:54 pm »

David, you asked for the industry references that prove your position wrong. Note how they all make reference to the act of human perception of the image with phrases like "appear reasonably sharp"....

To quote from Ansel Adams book "The Camera", chapter 5 "Lenses", page 48.

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We can achieve critical focus for only one plane in front of the camera, and all objects in this plane will be sharp. In addition, there will be an area just in front of and behind this plane that will appear reasonably sharp (according to the standards of sharpness required for the particular photograph and the degree of enlargement of the negative). This total region of adequate focus represents the depth of field.

and again on page 49...

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We must remember that the depth of field relates to an acceptable degree of sharpness; in actuality only the plane focused upon is truly sharp. Acceptable sharpness is also affected by the degree of enlargement of the negative and the distance from which the final print is viewed. An enlargement that looks well at 5 feet might be definitely unsharp at reading distance. Standard depth of field tables and scales are all based on certain assumptions regarding these factors.

To quote from a white paper on Zeiss's website titled  "Depth of Field and Bokeh" by H. H. Nasse, page 3...

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Depth of field is based on the acceptable blurriness and is therefore essentially based on arbitrary specifications.

and on page 19 it goes on to discuss the circle of confusion (text bolded by original author)...

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Depth of field is the result of an arbitrary specification, or rather it depends on the viewing conditions. Whether we tolerate a small or large amount of blurriness has no influence on the fundamental characteristics of the depth of field.

The human eye will not perceive any loss of sharpness in an image if the power of the eye is the only thing determining which smallest details can be recognized. On the other hand the eye will perceive an image as blurry if the eye is capable of seeing significantly more than is shown. The resolution that the eye can recognize must be the benchmark.

and on page 20...

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The depth of field is therefore a rather fuzzy dimension that depends heavily on the viewing conditions.

And to revisit the link from Norman Koren that you posted earlier in the thread as somehow supporting your position....

Quote
The depth of field (DOF) is the range of distances between sf and sr, (Dr + Df ), where the circles of confusion, Cf and Cr, are small enough so the image appears to be "in focus."

If you scroll to the bottom of the Norman Koren page, he posts a link to Bob Atkins's articles on Technical Optics. I know you have dismissed Bob as a purveyor of misinformation, but apparently Norman Koren does not believe so. In his article on depth of field, Atkins defines depth of field as follows:

Quote
Let's try to define depth of field. The usual definition runs something like this:

"The region over which objects in an image appear sharp".

While there is some truth in this, there's also some confusion - and some untruth too! Let's try a more accurate definition:

"The depth of field is the range of distances reproduced in a print over which the image is not unacceptably less sharp than the sharpest part of the image".

This definition contains some important points.

    * First, DOF relates to a print or other reproduction of an image. It's NOT an intrinsic property of a lens. If you put a lens on an optical bench you can measure focal length, you can measure aperture, but you can't measure depth of field. Depth of field depends on some subjective factors which I'll discuss later.
    * Second, note the phrase "not unacceptably less sharp". All parts of an image which come from outside the focal plane of the lens are blurred to some extent. Only one plane is in focus. As you move away from that plane things get less sharp. The depth of field limits are where the loss of sharpness becomes unacceptable - to a "standard" observer.
    * Third, note the phrase "..not unacceptably less sharp than the sharpest part of the image...". This covers the case of a pinhole camera. Such a camera has a very, very large depth of field (almost, but not quite infinite). However none of the image is sharp. The depth of field is large because all the image is equally blurred!

and finally, to quote from our host Michael Reichman on his article on this very website regarding Depth of Field...

Quote
You can't understand Depth of Field until you understand COF (Circle of Confusion). The human eye has a finite ability to see fine detail. This is generally accepted as being 1' (minute) of arc. Translating this to the practical world, this means that at a normal reading distance the smallest object that a person with perfect eyesight, under ideal conditions can see is 1/16mm in size. If you place two dots smaller than this next to each other they will appear to be just one dot.

...

Keep in mind as well that viewing distance plays a part in this. We're intimately dealing with the eye's inherent ability to discern detail, and obviously the farther away we are when we view a print, the larger the acceptable COF can be.

...

Depth of Field (DOF)

With an understanding that COF is a human imposed parameter that varies according to the manufacturer's whim and the vagaries of human perception we can now look at what is meant by Depth of Field. This is strictly an optical phenomena; and once a COF is applied no discretion is allowed.

Definition: "The area in front of and behind a focused subject in which the photographed image appears sharp".

Now that we understand what Circle of Confusion means we can see that this definition of Depth of Field means that this is the range in front of and behind the subject focused on that will appear sharp within the limits of the applied COF. In other words, you 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.


David, all of this information is from respected accurate sources, and it clearly shows that your understanding of what the term "Depth of Field" means is wrong.

Depth of Field is calculated from a Circle of Confusion that represents the ability of the human eye to see what is sharp vs blurry in a given set of viewing conditions. The choice of that CoC is the key variable in all of this, because it is what gives the calculations any meaning in the real world. CoC is not a measurement of pixel size or film grain size, it is a measurement of human visual acuity. Because of this...

DOF is a perceptual measurement, not a fixed standard.
You cannot have DOF apart from perceiving the image in some way.
That act of perceiving the image and the conditions that you do so under (ie print size, viewing distance) affect depth of field.
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ggriswold

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Re: Newbe question 35mm vs MF and dof
« Reply #84 on: January 03, 2011, 09:47:30 pm »

Talk about circles of confusion!  Found them right here.
Agree to disagree and go take some pictures already.
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David Klepacki

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Re: Newbe question 35mm vs MF and dof
« Reply #85 on: January 04, 2011, 01:31:34 am »

Yes, the relationship between DOF and CoC is clear cut and easy to understand.  What is causing the misunderstanding here is the context in which these terms are applied.  In the classical physics of optics, DOF is defined as a distance in the three dimensional world that is determined by near and far points of acceptable sharpness according to some value of CoC in the process of rendering via a lens.  If we are talking about the act of human vision, whether looking through a lens or not,  then a CoC value would indeed be based on the individual's ability to distinguish acceptable sharpness, and would be a function of his/her subjective ability to do so.  If we are instead using a photographic film to render the three dimensional world, then a CoC value must be used that is relevant to the nature of the film (e.g., grain size) and no longer base it on our biological and subjective vision.  This CoC will most likely be different than that of our human vision.  So, due to the different CoC value, you would indeed get a different DOF as I think we all agree.  And, as also pointed out above, a DOF calculator would indeed show this based on the different CoC input values.

The issue at hand regards the notion of whether DOF changes when talking about print size or screen projections or viewing on a monitor.  These things cannot change the DOF.  How can they, when DOF is measured in a third dimension that is no longer present?   I agree that you can simulate perceptual changes to DOF by manipulating the captured image in its two dimensional form, but the only actual DOF of the image is the one that is determined by the film that rendered the original scene from three dimensional space via a lens, and that can never change according to the physical laws of optics.  Furthermore, the CoC of this image on film indeed can be determined objectively (i.e., "fact") by measuring its grain size.  Whereas, the CoC that may be associated with human vision such as the perceptual interpretation of a print can be entirely subjective as Jeremy points out above.  (For the digital equivalent of establishing objective CoC values, you can again consult the spreadsheet from Alpa that I presented earlier as reference, where Alpa has done exactly that.)

Therefore, the ability to simulate perceptual subjective changes to DOF by post-manipulating the actual captured image (i.e., printing, projecting, blurring, etc.) does not amount to claiming that the DOF of the captured image depends on its print size.  In much of the popular photographic literature, this distinction is seldom articulated, probably for the same reasons why it is being debated at such length in this thread.  However, you would never find any lack of such distinction in a physics textbook on optics.
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #86 on: January 04, 2011, 02:01:57 am »

This is starting to head into the realms of trees falling in forests. The physicist would argue that of course it makes a noise, whereas the philosopher will argue that no human ear heard it so it has no meaning.

I am also concerned by your use of the term 'acceptable sharpness' mixed up with notions of classics optics. It seems to me that, unless someone sees the image, any DOF notions are meaningless.

I do take your point about the CoC of a sensor or film emulsion providing a kind of baseline CoC number but again, given that the sensor can itself resolve far more than the eye, this sort of CoC (around 0.02mm) is not really useful. You are really talking about resolution which then needs translating to some sort of viewable object.

Without a viewer surely all this talk about DOF is meaningless. Remember, we are talking about photography here not theoretical physics - to argue from such a position is to fall back on the rhetorical sleight of hand called  'moving the goal posts'. :-)

UPDATE.

After further thought, and a beer, I have found a flaw i your reasoning.

You are basing your position on this:

"In the classical physics of optics, DOF is defined as a distance in the three dimensional world that is determined by near and far points of acceptable sharpness according to some value of CoC in the process of rendering via a lens.  If we are talking about the act of human vision, whether looking through a lens or not,  then a CoC value would indeed be based on the individual's ability to distinguish acceptable sharpness, and would be a function of his/her subjective ability to do so."

It's not a case of 'if' we are talking about the act of human vision, of course we are talking about this. To consider anything else is meaningless. And the biggest hole in your logic is the term 'acceptable sharpness' - in your 'classical physics' model; if the resolution of the sensor defines the CoC, then there can be no room for 'acceptable' - it's a fixed number

Furthermore, your use of a CoC number for a sensor as a basis for measurment of DOF is simply wrong - the term is used in the context of resolution of fine details at the level of the sensor/film, not DOF. You are mixing up CoC as relating to resolution and CoC as relating to DOF - they are both measures of the threshold of sharp/blurred but one relates to a sensor's ability to resolve detail and the other relates to a viewer's ability to resolve detail on a print (or loupe or screen etc). CoC is simply a size, it can be applied to different systems - you are mixing them up. The proof of this is that the quoted CoC for a decent hi-res sensor is about 0.015mm, give or take. The CoC for human vision is about 0.1 - 0.2mm at 25cm. This is about 15 times more.

Sorry David, I thought you were on to something that might have been correct in certain theoretical circumstances but unfortunately you are wrong in your use of the CoC term and everything follows from that.

« Last Edit: January 04, 2011, 02:44:08 am by Nick Rains »
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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #87 on: January 04, 2011, 02:26:33 am »

In the classical physics of optics, DOF is defined as a distance in the three dimensional world that is determined by near and far points of acceptable sharpness according to some value of CoC in the process of rendering via a lens.  If we are talking about the act of human vision, whether looking through a lens or not,  then a CoC value would indeed be based on the individual's ability to distinguish acceptable sharpness, and would be a function of his/her subjective ability to do so.  If we are instead using a photographic film to render the three dimensional world, then a CoC value must be used that is relevant to the nature of the film (e.g., grain size) and no longer base it on our biological and subjective vision.  This CoC will most likely be different than that of our human vision.  So, due to the different CoC value, you would indeed get a different DOF as I think we all agree.  And, as also pointed out above, a DOF calculator would indeed show this based on the different CoC input values.

The issue at hand regards the notion of whether DOF changes when talking about print size or screen projections or viewing on a monitor.  These things cannot change the DOF.  How can they, when DOF is measured in a third dimension that is no longer present?   I agree that you can simulate perceptual changes to DOF by manipulating the captured image in its two dimensional form, but the only actual DOF of the image is the one that is determined by the film that rendered the original scene from three dimensional space via a lens, and that can never change according to the physical laws of optics.  Furthermore, the CoC of this image on film indeed can be determined objectively (i.e., "fact") by measuring its grain size.  Whereas, the CoC that may be associated with human vision such as the perceptual interpretation of a print can be entirely subjective as Jeremy points out above.  (For the digital equivalent of establishing objective CoC values, you can again consult the spreadsheet from Alpa that I presented earlier as reference, where Alpa has done exactly that.)

Therefore, the ability to simulate perceptual subjective changes to DOF by post-manipulating the actual captured image (i.e., printing, projecting, blurring, etc.) does not amount to claiming that the DOF of the captured image depends on its print size.  In much of the popular photographic literature, this distinction is seldom articulated, probably for the same reasons why it is being debated at such length in this thread.  However, you would never find any lack of such distinction in a physics textbook on optics.

So essentially, your contention is this... If I shoot an image on Velvia, it has less Depth of Field than if I shoot an equivalent image on Tri-X because  Velvia has finer grain.  And if I shoot a 21 megapixel full frame image (Canon 1Ds III) it has more depth of field than if I shoot the same photograph as a 25 megapixel full frame digital image (Nikon D3x) because the D3x has more megapixels.

To me, this seems like a nonsensical postion to take. Essentially, your argument is that depth of field is based upon focal length, aperture, subject distance, and the ultimate resolution of the format that the particular photograph is captured on. Can you please cite even one example of a textbook on "classical optics" that uses these four metrics (specifically the fourth) as a basis for Depth of Field?
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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #88 on: January 04, 2011, 02:36:57 am »

And given the number of times that I've posted in this thread despite my better judgment, and the fact that my wife is currently calling me to bed, I think it only fitting that I post this...

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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #89 on: January 04, 2011, 02:46:42 am »

LOL, excellent!

And I too have probably said my piece. As someone earlier butted in - agree to disagree! Actually I'll agree that someone else is disagreeing.

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Nick Rains
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #90 on: January 04, 2011, 02:55:00 am »

So essentially, your contention is this... If I shoot an image on Velvia, it has less Depth of Field than if I shoot an equivalent image on Tri-X because  Velvia has finer grain.  And if I shoot a 21 megapixel full frame image (Canon 1Ds III) it has more depth of field than if I shoot the same photograph as a 25 megapixel full frame digital image (Nikon D3x) because the D3x has more megapixels.

Ooh boy, I'm going to get into trouble for this but actually, this is the case, and it's not really what David is saying. Higher res systems do have less DOF for all sorts of reasons. You can make bigger enlargements before being resolution limited, this magnifies the capture CoCs even further, hence less DOF.

Running for cover...
 ;D
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Bart_van_der_Wolf

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Re: Newbe question 35mm vs MF and dof
« Reply #91 on: January 04, 2011, 03:59:28 am »

If we are instead using a photographic film to render the three dimensional world, then a CoC value must be used that is relevant to the nature of the film (e.g., grain size) and no longer base it on our biological and subjective vision.

That is the basis of your flawed argument. While the capture medium fixes resolution, it's not doing anything to DOF while COC does. The error in your reasoning is also demonstrated by magnifying an image, it will show shallower DOF than at the capture size which proves your theory is wrong.

Quote
The issue at hand regards the notion of whether DOF changes when talking about print size or screen projections or viewing on a monitor.  These things cannot change the DOF.  How can they, when DOF is measured in a third dimension that is no longer present?

That's also wrong. The by now fixed resolution in the capture medium will still be viewed 3-dimensionally, i.e. by varying angular resolution as a function of magnification and distance (fundamental parts of the COC determination).

Cheers,
Bart
« Last Edit: January 04, 2011, 11:08:44 am by BartvanderWolf »
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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #92 on: January 04, 2011, 10:42:36 am »

Ooh boy, I'm going to get into trouble for this but actually, this is the case, and it's not really what David is saying. Higher res systems do have less DOF for all sorts of reasons. You can make bigger enlargements before being resolution limited, this magnifies the capture CoCs even further, hence less DOF.

Running for cover...
 ;D



Of course...  but you have to make big prints or view the image at a large size for that to be true.  And as David has clearly established (I'm speaking tongue in cheek here) depth of field doesn't depend on print size, because it's fixed at the moment of capture onto the photographic medium.  Depth of Field is fixed, because the CoC must be selected based on the resolution of the medium it's captured on. Only then do you get the true depth of field!

All those poor D3x owners, running around taking pictures and not realizing they could have had more depth of field if they had just bought a D700 instead.
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David Klepacki

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Re: Newbe question 35mm vs MF and dof
« Reply #93 on: January 04, 2011, 08:59:56 pm »

Sorry David, I thought you were on to something that might have been correct in certain theoretical circumstances but unfortunately you are wrong in your use of the CoC term and everything follows from that.

Nick,

I think you almost got it, but somehow you managed not to trust your own intuition.  I used the term "acceptable sharpness" above because historically that is how DOF was described for the optics of human vision before photography was even invented, and it is still used today in various fields (e.g., optometry).  In these situations, CoC is indeed characterized by human visual characteristics involving "acceptable sharpness" such as acuity and the various viewing conditions as already pointed out.  With the invention of photography, it became possible for film to quantify DOF into an objective value based on the resolution of its emulsion, rather than the subjective visual ability of any one person.  To answer the other question posted here about variances in film, yes different film grain sizes mean different CoC values and likewise different DOF.  It is no different for digital cameras.  Digital cameras having different pixel sizes means having different CoC values for their rendering, which means having different values for the resulting DOF.  Once again, I cite the spreadsheet by Alpa that explicitly shows how Alpa views objective CoC values that vary with pixel size and corresponding capture device.

Let me try one more time and hopefully my position will become more clear.  DOF is always and only defined in a three dimensional space and in relation to an optical axis.  So, by viewing with your eyes a print, or a projected transparency on a screen, or a jpeg image on a monitor, etc., the DOF of all these objects as you look at them squarely must be essentially zero, since they are basically 2D planar objects along your axis of sight.  For example, I can take a photograph of the Sydney Opera House and the Harbour Bridge together within the same DOF, which is something probably around 3 km (depending on the lens being used, my location and the CoC of the film).  However, once this scene has been rendered into its 2D form, what is now the optical axis being used to view it as a print?  The optical axis is now along your eyes looking at the print, and so the DOF in this situation can be nothing other than zero, since you are looking at a piece of paper and not the actual Sydney Opera House.  Therefore, when you talk about "depth" in a photograph, as you correctly indicated in one of your previous responses, you are only talking about the illusion of depth perception.  This perception of "depth" cannot be said to be the DOF of the scene that was captured, since again DOF is a distance defined by the laws of optics in three dimensional space.

Yes, the perceived depth in a print can be related to the DOF of the underlying captured image, but the converse is not true and they are not the same thing.  The perceived depth in the print may not only involve the subjective visual dependencies that have already been identified in this thread, but it can also depend on a wide variety of other things via software manipulation such as selective blurring or sharpening, neither of which would change any CoC that may be associated with the print (or that of my eyes).  

As an example, suppose I make a straight print of a digital image.  Now suppose I slightly but noticeably blur only small selective parts of this same image using a Photoshop layer prior to creating a second print having the exact same size and under identical viewing conditions as the first print.  All aspects of this second print are the same as the first print, except that it will have an illusion of slightly reduced depth due to my selective blur.  None of the printing characteristics have changed, ink has not changed, paper has not changed, enlargement factor and print size have not changed, raw image file has not changed, viewing distance has not changed nor anything associated with the viewing conditions.  Yet, I am able to produce for you a second print which is identical in every way to the un-blurred first print, except for its slightly reduced perceived depth.  And, the blurring layer is carefully done using special brushes and cloning procedures to ensure that it cannot be claimed that I have changed the CoC of anything.  You already agree that DOF must depend on a CoC.  So, how do you now explain how these two prints can have different "DOFs" but without having different CoCs under otherwise identical conditions?

The answer is that you can't.  Under otherwise identical conditions, only perceived depth in the prints can be different without having a different CoC.  This is because the digital printing process may include masks or filters that can change the perceived depth in the resulting print without changing any CoC.  This cannot be true for DOF according to the physical laws of optics, and the DOF can only change if the CoC changes (all else being equal).  Simply put, the printing process does not play any role whatsoever in the definition of DOF.  Certainly, printing considerations are not needed when computing the DOF for lens systems such as eyeglasses.

Therefore, and my point in this entire thread, the DOF associated with a captured image cannot depend on any particular print of it.  The CoC used to define DOF must be that which is associated with the film or digital sensor used in the capture rendering process, and not the printing process.
« Last Edit: January 04, 2011, 09:03:33 pm by David Klepacki »
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #94 on: January 04, 2011, 09:59:25 pm »

You say your whole point is this: "The CoC used to define DOF must be that which is associated with the film or digital sensor used in the capture rendering process, and not the printing process."

Unfortunately this statement is totally contradicted by your referenced Alpa document:

I quote "The CoC (circle of confusion) is not a fixed value. DOF scales on lenses typically use a standard CoC based on image size (sensor or film) and a standard printsize of approximately 8x10 inch. In reality the CoC depends mainly on image size, enlargement, viewing distance (respectively the resolution of the human eye) and diffraction effects."

And reading on, further down the DOF page on the Alpa document (which is very useful actually, thanks for the link) you will see the section titled CoC Calculations - this is how you work out what CoC to use for your DOF calculations - and what do you see there? I see Print Width, Height, Distance (and even Eyesight) specifications related to the size of the initial image capture. And even more interesting, the results are displayed as a Magnification factor, a calculated COC, a Standard CoC and a High CoC.

If the CoC of the sensor is the only factor determining DOF then, by your assertion, it's a fixed number. Unfortunately Alpa, and all the DOF calculators, disagree.

One final nail in your theory's coffin is actually using a DOF calculator and plugging in the CoC figures that you insist should be used, based on the sensor size according the Alpa document. It shows that my Leica S2 with it's 70mm lens should have a DOF of about 10cm at 150cm feet using f2.5 using a CoC of 0.045. Trust me when I say this is totally incorrect - you can hardly get the ends of the eyelashes and the face of the eyeball sharp together in a head and shoulders portrait - the DOF of the capture (as displayed in a nice big 20"x30" print) looks to be about 1cm or less. Interestingly enough, in a small 5x7 test print, pretty much everything except the ears looks sharp. Go figure! OTOH if I plug in a CoC calculated back from the print size (0.01mm) I get a result of 1cm feet which agrees with real world experience.



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Nick Rains
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David Klepacki

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Re: Newbe question 35mm vs MF and dof
« Reply #95 on: January 04, 2011, 10:16:18 pm »

Aha!  The Alpa spreadsheet contains errors.  My bad for not checking everything thoroughly!  I was only trying to find a quick online reference where objective CoC values could be found.  Shame on Alpa!!

However, my argument is indeed 100% correct and valid.  The correct CoC for any Bayer digital sensor (without AA filter) is the following:

CoC = 2.0 * sqrt(2.0) * (pixel_width)

The pixel width of the S2 sensor is 0.006mm.  So, this would fix the CoC of the S2 to be roughly 0.017mm.  Please try this value in your DOF calculator.

« Last Edit: January 04, 2011, 10:27:17 pm by David Klepacki »
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #96 on: January 04, 2011, 10:25:30 pm »

OK, did that and no, wrong. Result is 3.75cm which is ludicrous. 

Interesting that Alpa is wrong...I'd have thought they were a fairly authoritative source, you even used them to confirm your theory. Or maybe they are not actually wrong...

Anyway, the only way the DOF calculator is even close is to use a CoC of about 0.005mm - 0.01mm. I can even show you a 100% crop of the image I am using as an example. The DOF is almost non-existent, under no circumstances is it 3.75cm.

Back to the drawing board.
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Nick Rains
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David Klepacki

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Re: Newbe question 35mm vs MF and dof
« Reply #97 on: January 04, 2011, 10:41:23 pm »

Nick,

The DOF and CoC formula have been verified by myself and many other people over many years.  How accurate are your distance measurements?  Are you introducing any vibration at all, perhaps by hand-holding the camera?  

If you are telling me that the S2 yields such a drastic difference to the calculated DOF when using a heavy tripod, mirror lockup and accurate distance measurements, and using a static subject, then that only means that the S2 focal plane shutter is introducing much more vibration than it should.  Shame on Leica in that case.

If there is any blur that is manually introduced into the system (whether by target or photographer), then of course this blur is not figured into the DOF calculators, and the blur that is introduced will reduce the depth that you perceive in the image.

Update
--------
My colleagues inform me that the CoC formula actually has the following range for Bayer sensors without AA filters:

1.4 * (pixel_width) <= CoC <= 2.0 * (pixel_width)

The range is supposedly due to the variance in raw conversion quality, which supplies 2/3 of the missing pixels in a digitally captured image, as well as the lens quality.

However, as I mentioned above, any blur that is introduced into the capture is not taken into account by any DOF calculator.
« Last Edit: January 10, 2011, 01:05:20 am by David Klepacki »
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #98 on: January 04, 2011, 11:02:49 pm »

I'll not going to take offense at you questioning my techniques... I have been a pro photographer for 28 years and I do know what I'm doing. I was going to post a crop of the image in question but I can't be bothered as you will come up with some other excuse as you how you are right and everyone else is wrong.

Your CoC formula is dubious - looks like sloppy maths to me. Why express it the way you did? It just means 2.818 times the pixel pitch. Who claims this is the correct formula 'over many years'? All your references so far have proved you wrong, so you claim they are mistaken. LOL!

Sorry mate, you are wrong. Nothing you have said has been confirmed elsewhere (at least nothing you have referenced), no one agrees with you and you resort to claiming everyone else must be wrong. It was a good discussion though, so thanks for the entertainment.

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Nick Rains
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David Klepacki

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Re: Newbe question 35mm vs MF and dof
« Reply #99 on: January 04, 2011, 11:13:46 pm »

Nick, I am sorry that you feel this way. 

I have not invented anything new here, and I have only cited well known DOF formulas.  I have not disagreed with any of the DOF formulas that you or anyone else here has linked or referenced.  Please show me where you think I presented a DOF formula that is different.

As for the CoC formula I presented, it is the same as also used by authors of a well-known article published here on Lula:

http://www.luminous-landscape.com/tutorials/resolution.shtml

So, if you want to claim that these gentlemen are guilty of sloppy math, that is your opinion.

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