# How to determine diffraction limit for a lens?

Started Jun 28, 2011 | Discussions thread
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Of course not. KM had it exactly right...

KM Legacy wrote:

Print size and viewing distance have a lot to do with DOF. Since nothing is really in focus except in a single plane, and things in front and behind vary continuously in their blurriness, how can anyone define DOF? Focus does not abruptly stop at the limits of DOF. It's just arbitrarily defined on the basis of the human eye's ability to detect blur under specified conditions which somebody considers to be "normal" or "typical." If you do the equivalent of pixel-peeping by moving close to a print on the wall, what is "acceptably" sharp is different from what is the case if you stand 5 feet away.

Mostly Lurking replied:

Perceived DOF, yes. DOF itself, no---it's defined. [nt]

KM Legacy replied:

DOF is defined according to how it is perceived under standardized conditions; i.e. viewing a print of a certain size at a certain distance, what does a normal human eye perceive as acceptably sharp? There is no real mathematical definition of DOF. All the very precise-looking tables of DOF which are published for a given lens at various apertures are based on the assumptions I just stated above. If the assumptions or conditions were changed, e.g. by changing the distance at which the print is viewed, then what is "acceptably sharp" would change, and therefore DOF would also change.

Mostly Lurking replied:

Of course not. KM had it exactly right.

Now, you have a history of denying pretty much anything anyone tells you, William. So, i'm going to go to a pretty high level source on this one...

Here's a nice 45 page publication on how DOF is calculated (and how Bokeh can be both calculated and evaluated) from Dr. Hubert H. Nasse of the Carl Zeiss camera lens division.

http://www.zeiss.de/C12567A8003B8B6F/EmbedTitelIntern/CLN_35_Bokeh_EN/ \$File/CLN35_Bokeh_en.pdf

The explanation of how the common DOF circle of confusion criteria is derived starts on page 19.

All of the curves and tables shown so far have been calculated assuming a circle of confusion diameter that fits 1,500 times into the diagonal of the image. But we must explain why this size is so often chosen and why we should sometimes choose another one. 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.

I won't quite the entire document. Paragraphs 5 and 6 give examples of DOF for two other print sizes. Page 20, paragraph 1 gives an example of a DOF criterion from the 1950s.

Oh, and welcome back.

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Rahon Klavanian 1912-2008.

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