Newbe question 35mm vs MF and dof

[Bart_van_der_Wolf]

My only point here has always been that DOF is fixed at point of capture (whether film or digital), and it does not change thereafter.

Hi David,

Maximum resolution is fixed at the moment of capture, not DOF. At the moment of capture, the sensel pitch (= discrete sample in the spatial domain) determines the maximum achievable resolution. Any subsequent magnification of that captured image will reduce that same maximum resolution (usually expressed in cycles/mm). Therefore, the intended use (= magnification) of the image data will impact the maximum achievable resolution. When the maximum achievable resolution exceeds our COC criterion for a specific use, the maximum DOF is impacted (= limited). The COC criterion is therefore the implicit limit on resolution, and the COC criterion is variable (!) (because it depends on the end use (= magnification))

So, magnification impacts DOF (assuming its common definition of limited acceptable resolution), as does viewing distance (= magnification).

For a more authoratitive explanation of DOF, I suggest you read:
http://toothwalker.org/optics/dof.html
and for a more mathematical underpinning:
http://toothwalker.org/optics/dofderivation.html

Cheers,
Bart

[David Klepacki]

OK, so what is Depth of Field? Give me your definition of what you understand this term to mean.

If you examine any correct definition I think you will find that everything that has been explained to you is in fact the case. Any other interpretation must be derived from a different definition and thus we are talking about different things - probably more semantics than anything else.

I am not creating any unique definition for Depth of Field.  I am in total agreement with the correct definition, such as the one found here for example:
http://www.normankoren.com/Tutorials/MTF6.html

The misunderstood notion that I am trying to correct is that DOF is defined only in the context of rendering a three dimensional scene onto a two dimensional capture medium.  Once that process is completed, the DOF is fixed and does not change.  You can view the results of this rendering in various ways such as with monitors or with prints of various sizes, but the DOF of the captured image does not change because of these various ways in which you choose to view it.  The post-capture viewing process involves the notion of viewing resolution, by which images can be made to appear sharper or softer depending on the underlying resolution of the medium being viewed, but the DOF of the image remains the same and is defined by the characteristics of its capture.

[Nick Rains]

I am not creating any unique definition for Depth of Field.  I am in total agreement with the correct definition, such as the one found here for example:
http://www.normankoren.com/Tutorials/MTF6.html

Koren makes no mention of print size one way or the other, but one of his recommended links does:

http://bobatkins.com/photography/technical/digitaldof.html



"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!

An important thing to note is that depth of field is NOT what some people think it is, i.e. a well defined zone over which everything is in sharp focus. Some people seem to have the impression that an image has two zones. In focus and out of focus. In fact there is only one point (actually plane) in focus. Everything else is out of focus to some extent."



This seems quite clear.

[Ray]

Circle of Confusion is generally defined as as the largest blur spot that will still be perceived by the human eye as a point.

I've always thought that the true definition of Circle of Confusion is 'a group of photographers sitting around in a circular fashion, discussing DoF matters'. 

[David Klepacki]

Maximum resolution is fixed at the moment of capture, not DOF.

Koren makes no mention of print size one way or the other, but one of his recommended links does:

http://bobatkins.com/photography/technical/digitaldof.html

Bart, You are perpetuating the major confusion between viewing resolution and DOF (and Bob Atkins is also guilty of this, Nick).  Yes, maximum resolution is also fixed at the moment of capture due to the fundamental pixel pitch of the sensor or grain size of the film.  However, this does not change the definition of DOF, even according to your own website references.

It is viewing resolution that is impacted by magnification and viewing distance, not DOF. 

Once more, DOF is defined and fixed only at the point of capture in the context of rendering a three-dimensional scene onto a two-dimensional medium.  All else thereafter is about viewing resolution.

[Nick Rains]

Once more, DOF is defined and fixed only at the point of capture in the context of rendering a three-dimensional scene onto a two-dimensional medium.  All else thereafter is about viewing resolution.

Right back atcha...

DOF relies on CoCs to have any meaning, just look at any DOF calculator. CoCs are whatever you define them to be, but always based on a viewer and is usually the size at which a point is seen as a disc. This obviously varies slightly depending on the viewer's visual acuity. The viewer also has to view something for DOF to have any real world meaning, therefore the print/screen is entirely relevant because the more you enlarge the image data the more 'points' that were below the threshold of the defined CoC become revealed as discs.

[Bart_van_der_Wolf]

Maximum resolution is fixed at the moment of capture, not DOF.

Bart, You are perpetuating the major confusion between viewing resolution and DOF

David,

You're wrong. I didn't mention viewing resolution here.

Yes, maximum resolution is also fixed at the moment of capture due to the fundamental pixel pitch of the sensor or grain size of the film.  However, this does not change the definition of DOF, even according to your own website references.

Then humour us some more, and do point out where Paul van Walree's site disagrees.

It is viewing resolution that is impacted by magnification and viewing distance, not DOF.

Then your definition of DOF differs from the accepted definitions. Which again raises the question, "What is your definition of DOF"?

Cheers,
Bart

[Sheldon N]

I think David already said what he believes the proper definition of DOF is, as indicated in Norman Koren's site on the link he posted. I'll quote it here so he doesn't have to rephrase it himself.

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." The standard criterion for choosing C (the largest allowable value of Cf and Cr) is that on an 8x10 inch print viewed at a distance of 10 inches, the smallest distinguishable feature is (allegedly) 0.01 inch. That was the assumption in the 1930's when film was much softer than it is today. At 8x magnification this corresponds to 0.00125 inches = 0.032 mm on 35mm film, close to the standard 0.03 mm used by 35mm lens manufacturers to calculate their DOF scales. If you've ever had a close look at a fine contact print from 4x5 or 8x10 film, you'll doubt that 0.01 inch feature size is a good criterion. Studies on human visual acuity indicate that the smallest feature an eye with 20:20 vision can distinguish is about one minute of an arc: 0.003 inches at a distance of 10 inches. But inertial prevails: 0.01 inch is universally used to specify DOF.

I've highlighted the key points in bold.

[Nick Rains]

mmm, yes, I guess this is why this discussion is kinda drifting into semantics. If you apply this definition word for word then you are in fact specifying a particular CoC, a print size and a viewing distance. In which case, yes the DOF is fixed at the point of shooting. No argument there, how could there be?

The point is that, obviously, once you change the viewing definitions then the DOF changes accordingly. Therefore DOF is related to print size etc.

Where David is coming from (I think) is that he is taking that definition as the only definition (or solution) of DOF, which is not the case - it's one of many equally valid solutions to the various equations, all of which depend on choosing an appropriate CoC.

[Bart_van_der_Wolf]

I think David already said what he believes the proper definition of DOF is, as indicated in Norman Koren's site on the link he posted.

Yet he is contradicting it with his own statements by disregarding the COC, hence the question to explain.

FWIW, here is the definition by Paul van Walree (http://toothwalker.org/optics/dof.html), which David seems to disagree with:
"Depth of field is defined as the range of object distances within which objects are imaged with acceptable sharpness.

A whole series of definitions can be found at Don Fleming's site, where a casual glance suffices to conclude that they all amount to the same thing. That is a welcome observation, because it means that there is consensus of opinion regarding the definition of DOF".

And Wikipedia (http://en.wikipedia.org/wiki/Depth_of_field) says a similar thing:
"In optics, particularly as it relates to film and photography, the depth of field (DOF) is the portion of a scene that appears sharp in the image. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions".

Unlike what David states sofar, acceptable sharpness, and what appears to be sharp clearly includes an observer.

Cheers,
Bart

[Sheldon N]

Yet he is contradicting it with his own statements by disregarding the COC

Quite true, and an amusing irony.  Exactly why I posted it.   

[David Klepacki]

DOF relies on CoCs to have any meaning, just look at any DOF calculator. CoCs are whatever you define them to be, but always based on a viewer and is usually the size at which a point is seen as a disc. This obviously varies slightly depending on the viewer's visual acuity. The viewer also has to view something for DOF to have any real world meaning, therefore the print/screen is entirely relevant because the more you enlarge the image data the more 'points' that were below the threshold of the defined CoC become revealed as discs.

Nick (and others),

Yes, DOF relies on CoC to have meaning.  However, the converse is not true.  In other words, not all CoC involve the concept of DOF.  This is where your thinking is going wrong.  Printers have a CoC that is associated with how finely its dots can be perceived by a viewer.  It cannot possibly have anything to do with DOF.  It is simply used to characterize the perceived resolution of whatever it prints.  For example, let's say I create a simple vertical 2D resolution chart on my computer by drawing horizontal lines closer and closer together from bottom to top, and then print it out as a jpeg.  Using whatever CoC you wish for the printer, what would you conclude about the DOF of my 2D chart when printed?  Obviously it is 2D and so it has no DOF.  

Furthermore, if I print my 2D chart smaller its lines will appear to be sharper, and if I print it larger and larger, those lines will appear to become softer and softer and the lines will blur together creating a narrower range of resolution.  However, at no point does the DOF become anything other than zero, since it is a drawing.  The apparent change in sharpness range is a result of your viewing resolution.

Hopefully, this example will clarify for you the difference between viewing resolution and DOF.  You cannot say that DOF depends on print size.  You can only say that its viewing resolution depends on print size.

[Nick Rains]

Printers have a CoC that is associated with how finely its dots can be perceived by a viewer.    It is simply used to characterize the perceived resolution of whatever it prints.  

Sorry David, (with all due respect to the enjoyable and ongoing civility of this thread) you are labouring under some serious misconceptions, as clearly demonstrated by this comment above.

CoCs and printers have nothing to do with one another. CoCs are merely an arbitrarily defined threshold of visible detail/sharpness and are entirely independent of any device - it's just a number. Printers do not have CoCs, they merely reproduce the data from the image, whether it's sharp or not. A printer's own ability to resolve details is (mostly) to do with ink-dots per inch - all of which are below the threshold of unaided-eye resolution.

[David Klepacki]

CoCs and printers have nothing to do with one another. CoCs are merely an arbitrarily defined threshold of visible detail/sharpness and are entirely independent of any device - it's just a number.

Nick, I am afraid it is not me who has misconceptions here.

A CoC can indeed have something to do with printing (as well as DOF).  Just take a look at the Wikipedia page on Circle of Confusion where they say
"The common criterion for “acceptable sharpness” in the final image (e.g., print, projection screen, or electronic display) is that the blur spot be indistinguishable from a point."  Here is the link:
http://en.wikipedia.org/wiki/Circle_of_confusion

In addition, even Alpa does not agree with you with regard to your claim that CoC is independent of any device.  Contrary to your belief, Alpa indeed finds CoC as being dependent on device.  For example, just take a look at their spreadsheet here where they associate various CoCs for different devices:
http://www.alpa.ch/dms/products/tools/alpa-comparable-focal-length-calculator/ALPA_CFL_Calc_V217B.xls

If you go back and think a little harder about the previous example I just gave you (with the printing of the 2D resolution chart of zero DOF), you should be able to see how the confusion between DOF and print viewing resolution arises due to the misunderstanding of the different meanings of their respective CoCs.

[Nick Rains]

Nick, I am afraid it is not me who has misconceptions here.

A CoC can indeed have something to do with printing (as well as DOF).  Just take a look at the Wikipedia page on Circle of Confusion where they say
"The common criterion for “acceptable sharpness” in the final image (e.g., print, projection screen, or electronic display) is that the blur spot be indistinguishable from a point."  Here is the link:
http://en.wikipedia.org/wiki/Circle_of_confusion

In addition, even Alpa does not agree with you with regard to your claim that CoC is independent of any device.  Contrary to your belief, Alpa indeed finds CoC as being dependent on device.  For example, just take a look at their spreadsheet here where they associate various CoCs for different devices:
http://www.alpa.ch/dms/products/tools/alpa-comparable-focal-length-calculator/ALPA_CFL_Calc_V217B.xls

If you go back and think a little harder about the previous example I just gave you (with the printing of the 2D resolution chart of zero DOF), you should be able to see how the confusion between DOF and print viewing resolution arises due to the misunderstanding of the different meanings of their respective CoCs.

I am pretty comfortable with my understanding of what a CoC is. And I'm pretty sure you are also clear on its empirical meaning in the sense that as an image 'point' becomes pregressively defocussed it eventually becomes recorded as a spot or disc (Airey Disc) covering a certain number of sensels. Once it covers 2x2 sensels it is usually considered degraded or 'out of focus'. This is part of the definition surrounding diffraction error etc but will serve here to make sure we are all on the same page regarding what a CoC is in its most basic form.

Your Wikipedia reference confirms everything we (Sheldon, Bart and myself) have been trying to explain, please consider the following section, in particular point two and three:

 "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 mm.

The common values for CoC may not be applicable if reproduction or viewing conditions differ significantly from those assumed in determining those values. If the original image will be given greater enlargement, or viewed at a closer distance, then a smaller CoC will be required. All three factors above are accommodated with this formula:
CoC (mm) = viewing distance (cm) / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement / 25
For example, to support a final-image resolution equivalent to 5 lp/mm for a 25 cm viewing distance when the anticipated viewing distance is 50 cm and the anticipated enlargement is 8:
CoC = 50 / 5 / 8 / 25 = 0.05 mm

Since the final-image size is not usually known at the time of taking a photograph, it is common to assume a standard size such as 25 cm width, along with a conventional final-image CoC of 0.2 mm, which is 1/1250 of the image width. Conventions in terms of the diagonal measure are also commonly used. The DoF computed using these conventions will need to be adjusted if the original image is cropped before enlarging to the final image size, or if the size and viewing assumptions are altered."

(My emphasis)

That's all we are saying, if you enlarge the image past it's original size (ie view or print it) you need to recalculate the DOF as the assumed CoCs are no longer valid.

Your CoC chart from Alpa merely defines the CoC of the device at the native size of the image, whether a piece of film or a digital sensor, not when it's enlarged. I admit my 'device independent' comment was misleading but the above reference, provided by you, should make my (our) meaning clear.

[David Klepacki]

Your Wikipedia reference confirms everything we (Sheldon, Bart and myself) have been trying to explain, please consider the following section, in particular point two and three: ...

C'mon Nick, The Wikipedia reference does not confirm Sheldon's Response #27 where he claims, "Without the print or monitor, DOF does not exist."  (I don't think even you believe that one.)  Nor does it confirm your own claim in your Response #53 where you say, "CoCs and printers have nothing to do with one another."  In fact, it directly contradicts that.   I was only trying to find a common definition for CoC that we can agree on.  Unfortunately, they make the error about DOF being changed if one crops the original captured image.  What actually happens is that by reducing the original captured image, any magnification factor will lead to reduced viewing resolution (but not reduced DOF) of the captured image.

OK, let me try another example.  Let's say I have captured an image on 6x6 positive transparency film.  When I view the image on a light table using a loupe, I can accurately gauge what the visible DOF appears to be at the point of its capture.  If I then place this same transparency into a slide projector and display it on a screen, do you claim that the DOF of my transparency has changed in any way?  Or, are you claiming that only its viewing resolution as seen on the screen is much less due to its magnification, but that the actual DOF on my transparency has not changed in any way.  If it is the latter case, then I think we are in agreement.

[Nick Rains]

What part of the Wikipedia quote do you disagree with? Everything we are trying to explain is right there.


DOF calculators require you to specify a CoC size. True.

Appropriate CoC sizes depend entirely on the intended size of the display or final enlargement (magnification). True.

Ergo, DOF depends on CoC size. OK?

DOF calculations therefore depend on the final size of the print. So many of the quoted sources state this quite explicitly, including all the ones you mentioned yourself - the Alpa spreadsheet says the same thing.

Wikipedia again...

"The common values for CoC may not be applicable if reproduction or viewing conditions differ significantly from those assumed in determining those values. If the original image will be given greater enlargement, or viewed at a closer distance, then a smaller CoC will be required. Smaller CoC equals less DOF according to any DOF calculator you choose.


I really don't know how to explain this any clearer. Sorry. Anyone else care to have a shot...?

[David Klepacki]

Nick,

I presented to you a clear scenario where I think it is possible for me to understand you.  So again, please answer my previous question about what you believe happens to the DOF of my transparency example above?  Do you claim that the DOF of my transparency has changed in any way by viewing it with the projector?  Or, are you claiming that only its viewing resolution as seen on the screen is seen to be much less due to its magnification, and that the actual DOF on my transparency has not changed in any way? 

[Nick Rains]

If you make a contact print of your tranny or just look at it with the naked eye (ie zero magnification) it will appear that pretty much everything is in focus because your naked eye lacks the resolution to correctly reveal the transition between the OOF areas and the point of true focus. Your eyes' resolution is a lot less than that of the film at this point and is the limiting factor. As you progressively enlarge the image to x2, x3 x4 etc, more and more areas that previously looked sharp will reveal themselves to be actually unsharp. The region of acceptable focus is now decreasing with the increasing enlargement and, by the definition of DOF, you now have less DOF as a result of enlarging the image.

That's all. Noting changes in the tranny, obviously. Just the increasing enlargement 'reveals' more and more OOF areas to the point where the full resolution of the tranny is reached (you can see grain) at which point the whole image will look progressively less sharp as it's further enlarged. The truly OOF areas are effectively obscured by the inability of your eye to see them - that's what CoCs measure. That's why the CoC must be defined in any DOF calculator by the final print size.

So, yes, you are correct to say that the actual depiction of OOF regions on the actual tranny (with respect to it's inherent resolution) has not changed. Having said that, can you now see why the DOF can be said to be print/display size related? Not so much in bigger prints, but smaller prints' lack of resolution actually obscures the OOF areas. This fits any definitions of DOF and so small prints with minimum enlargement can be said to have greater DOF than you will get in larger prints, up to the maximum captured in the first place.

I feel we are on the cusp of agreement here! Fingers crossed.

[Sheldon N]

I'll chime back in...

To piggyback on the negative on the light table analogy and what Nick said, think about how you view the transparency. Does using a stronger loupe change the negative? Of course not, but it does change what you are able to see to be in focus vs out of focus.

Another example would be from my days of shooting a 4x5 view camera. If I set up the camera and focus the scene using a standard 4x loupe on the ground glass, some portion of that scene will appear in focus and the rest will appear out of focus. If I leave everything else constant and simply now view the ground glass with an 8x loupe, I can now see that a much narrower portion of the image is actually "sharp" on the ground glass. Nothing about the camera or lens has changed and there isn't any film or even a photographic capture involved, but the DOF for those two different viewing scenarios was different.

I was using a bit of hyperbole when I said that apart from the print or image there is no DOF. The crux of what I mean is that apart from the act of viewing the image, (be it on the focus screen, ground glass, light table, monitor, print or any other way that you are able to see it) the concept of DOF has no meaning. It is just empty math.

If I had to summarize what I think the crux of David's argument is, I think he is saying that DOF should be defined as a fixed standard by choosing a CoC that represents the smallest possible size of detail that was captured on the native format (ie. one film grain, one pixel, etc) and calling this the native "DOF" of the capture.

However, this raises an interesting set of problems. If I shoot an image with a DSLR as a "Large JPG", then without changing anything with the camera/lens/scene I simply set the camera to "Medium JPG" and reshoot the same image, does it now have more DOF? It certainly has less resolution, and if you made a pair of comparison prints beyond the resolution limit of the "Medium JPG" file, the "Large JPG" file would start to have shallower DOF. But any other pair of smaller sized prints would look identical. You could do the same thing on film, shoot with a 4x5 view camera, one image on Tri-X (grainy) and another duplicate exposure on Velvia (finer grain, higher resolution). Does one have more or less DOF than the other?  Defining DOF based upon a CoC based upon the native capture of the photograph's film grain/pixel properties ultimately leads you down a path to where you are simply describing resolution.

What Nick, Bart, myself, and all the industry definitions I've ever come across agree upon is that Depth of Field is a perceptual measurement. It is, in simple terms, the measurement of what looks sharp and what doesn't. Because this is a measurement of what we see, it is constantly in flux. We want to make it a calculable answer, so we try to nail down each one of the variables with assumptions about standard print size, normal viewing distances, average human visual acuity, etc. until we get a final "number" that is the DOF. But in reality if DOF is perceptual and only exists at the moment we are seeing the image in the real world, it's never going to be a constant. We just take a measurement of a point in time and set of viewing conditions, and calculate the answer from that. 

I think everyone in this conversation has a good grasp on the fundamentals at hand. Where we are all hanging up is that we can't quite determine what David believes the definition of Depth of Field to be, and because he keeps making pronouncements about what is or isn't true about DOF (often in contradiction to what the rest of us believe to be accurate) the conversation keeps going on and on.

I think we could draw this to a close if David would answer two straightforward questions from his perspective.

1) What is the definition of DOF?
2) Assuming DOF is a fixed property (requiring a specific CoC to be chosen), how do you select what specific CoC to use, and why?