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.

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.

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.

Some comments:
Michael Reichmann actually said 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:


Ray,
You are getting into the situation I described earlier.
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.

Side-note: this article about Blur quality (Bokeh) and Blur quantity is extremely interesting, and may be of interest even for DOF fanatics here...

Olivier

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.

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.

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?

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.)

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.

[quote=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).

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.

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[I] the pixel size or more precisely, pixel pitch.

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.

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?



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?

Isn't the image size in the camera the format[I]?

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.

[quote=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.

[quote=gkramer,Oct 12 2006, 08:05 PM]
[quote=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.)


[/quote]

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.

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?

Well, whatever your personal system, the answer to the poster's question is clearly DOF does change with format size.

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.

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

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.

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.

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.

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.)

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).

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.

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.

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.

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)



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.


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!

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.

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

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.

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.

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?

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.

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.

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?

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.

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.

No Ray, to try to reason with you is nonsense. I'm done.

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.

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

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.

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

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."

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 .

Click to view attachment

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

Wiki entry - Depth of Field

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.