Choice for the Circle of Confusion?

[Pere Casals]

I'm starting a project to calculate my own DOF tables.

It is perfect having the own DOF tables, but anyway this can be complemented by a good DOF app:




Also think that calculated CoC / DOF are theoric and each lenses its particular behaviour. In cinematography they use through focus MTF charts.


IMHO it should be understood how DOF CoC is compared to the CoC of the lens in the perfect focus.

[MAubrey]

The original 0.033 for 35mm film assumes an 8x10 final print. So...like Dan says...

How much do you want to enlarge? That's what determines how fuzzy is too fuzzy.

[Bob Salomon]

The original 0.033 for 35mm film assumes an 8x10 final print. So...like Dan says...

And how far away will it be viewed at?

[Dan Fromm]

To summarize this discussion, in post #8 above Emmanuel Bigler told the OP, and very politely too, that it had asked too narrow a question. Emmanuel's answer provides a procedure for finding CoC given negative size, print size and the distance at which the print is to be viewed. CoC is clearly not a fixed number, one per film format. It depends on how much the negative is to be enlarged and the distance from which the print is viewed. Change either of these and the CoC will change. Ain't no one CoC that fits all situations.

From this it follows that in spite of the tremendous effort that's been put into DoF tables -- see A. A. Blaker's book Applied Depth of Field -- and calculators such as DOFMaster the enterprise is more than a little silly. Why calculate when you can (a) see a contact print's DoF on the ground glass and (b) see how enlargement will shrink it by swapping in a longer lens? Yes, I know, we already encumber ourselves with too much too heavy gear. That's what assistants are for.

[Bob Salomon]

To summarize this discussion, in post #8 above Emmanuel Bigler told the OP, and very politely too, that it had asked too narrow a question. Emmanuel's answer provides a procedure for finding CoC given negative size, print size and the distance at which the print is to be viewed. CoC is clearly not a fixed number, one per film format. It depends on how much the negative is to be enlarged and the distance from which the print is viewed. Change either of these and the CoC will change. Ain't no one CoC that fits all situations.

From this it follows that in spite of the tremendous effort that's been put into DoF tables -- see A. A. Blaker's book Applied Depth of Field -- and calculators such as DOFMaster the enterprise is more than a little silly. Why calculate when you can (a) see a contact print's DoF on the ground glass and (b) see how enlargement will shrink it by swapping in a longer lens? Yes, I know, we already encumber ourselves with too much too heavy gear. That's what assistants are for.

You can also see DOF at various magnifications on the gg depending on the magnification of the loupe that you use. However you would only see it in small parts of the gg that the loupe covers.

[Drew Wiley]

It seems he's having eyesight problems that make full use of a loupe unrealistic, Bob. Otherwise, all this circle of confusion talk could just be thrown out the window.

[Pere Casals]

You can also see DOF at various magnifications on the gg depending on the magnification of the loupe that you use. However you would only see it in small parts of the gg that the loupe covers.

Yes, but GG cannot display by far (because grain) all Image Quality that can be recorded on film, so we won't be able to judge the finest effects.

Emmanuel's answer provides a procedure for finding CoC given negative size, print size and the distance at which the print is to be viewed. CoC is clearly not a fixed number, one per film format. It depends on how much the negative is to be enlarged and the distance from which the print is viewed. Change either of these and the CoC will change. Ain't no one CoC that fits all situations.

Yes... first step is what Emmanuel pointed to reduce the conceptual complexity, if we consider a print size we also know what diffraction we may add to also extend DOF to not damage required IQ in the focus plane.

There is a balance, as we extend DOF to improve the rendering of certain spots we also stop the aperture, adding diffraction. Really, it's not that easy (or possible) to make an optimal shot if not knowing the print size,

Of course it's quite important to know what's the critical CoC depending on negative size vs print size. The enlargement factor tells how the "in negative CoC" also will be enlarged, so we only need to say what CoC we want on the print and dividing by the enlargement factor to have our CoC on film.

A problem is that often we don't know how large we'll print.


Then next step would be understanding how, for a very big print, we damage IQ in the focus plane as we extend DOF by stopping, in that case we should consider our lens performance.

And, of course, we have the Scheimpflug, which adds additional complexity to any calcualtion.


IMHO this is a very interesting matter, LF has insane IQ, but managing to have an optimal shot it's not that easy, anyway we may have an IQ overkill.

[Bob Salomon]

“Yes, but GG cannot display by far (because grain) all Image Quality that can be recorded on film, so we won't be able to judge the finest effects.”

What finest detail would bother you? By definition DOF is the “apparently” sharp area in front and behind the subject. That’s all it is.

[Pere Casals]

“Yes, but GG cannot display by far (because grain) all Image Quality that can be recorded on film, so we won't be able to judge the finest effects.”

What finest detail would bother you? By definition DOF is the “apparently” sharp area in front and behind the subject. That’s all it is.

Bob, the problem if that we won't see in what extent we damage ultimate IQ in the focus plane when stopping for DOF. Say we stop f/32, diffraction limit is 50cycles/mm but it also damages contrast at 25 cycles/mm. We won't see this in the GG, because GG graininess won't allow it.

Also if we have to inspect the GG at f/32 then we have a very dim image.


Also we may not be able to judge how sharp is the “apparently” sharp area, also because film records more than the GG allows to see.


A way to learn it without wasting film is attaching a DSLR (without lens) in the camera back, (I did it by fixing a macro extension ring in a Sinar lens board, and placing the resulting Nikon F mount in the Norma back.) A DX consumer DSLR sports 250 pix/mm which is good to explore DOF vs focus plane IQ.

That way does not substitute tables, apps and calcultions, but's a first hand direct view of the real effects.

[Bob Salomon]

Bob, the problem if that we won't see in what extent we damage ultimate IQ in the focus plane when stopping for DOF. Say we stop f/32, diffraction limit is 50cycles/mm but it also damages contrast at 25 cycles/mm. We won't see this in the GG, because GG graininess won't allow it.

Also if we have to inspect the GG at f/32 then we have a very dim image.


Also we may not be able to judge how sharp is the “apparently” sharp area, also because film records more than the GG allows to see.


A way to learn it without wasting film is attaching a DSLR (without lens) in the camera back, (I did it by fixing a macro extension ring in a Sinar lens board, and placing the resulting Nikon F mount in the Norma back.) A DX consumer DSLR sports 250 pix/mm which is good to explore DOF vs focus plane IQ.

That way does not substitute tables, apps and calcultions, but's a first hand direct view of the real effects.

Why do you keep reinventing the wheel? Photographers controlled DOF accurately and easily well before digital came about.
Their are numerous manual DOF devices for calculating and controlling DOF like the Rodenstock pocket calculator, DOF control rings on Sinar and Linhof cameras and the Rodenstock pocket one also computes Scheimpflug.

More importantly these systems calculate the proper position for the rear standard to focus on the correct point for maximum DOF. Eliminating the beginner mistake of focusing on the subject and losing most of the possible DOF and then stopping down too far to regain the lost area but ending in diffraction!

Get your self the pocket Rodenstock and do it easily, no batteries, no electronics, no rulers needed!