Moving Away from Circle of Confusion...

Without the Confusion

Photographers adopt techniques that give predictable results. Experienced photographers know what works for them. So if a new and superior way of approaching one’s photography is presented (as is done via the OptimumCS-Pro, TrueDoF-Pro and FocusStacker apps), any transition to a different way of thinking needs to be as simple and easy as possible.

Consider  the fact that the above mentioned apps all substitute the concept of “blur spot diameter” for the concept of “circle of confusion.” How different are these? Well, we could say that CoC is simply a geometric construction, while “blur pot diameter” refers to the combined effects of defocus and diffraction. But leaving it at that doesn’t cover what a photographer really needs to know. How do we answer a photographer who ask the following question:

  1. Through many years of experience, I know what value for CoC to use for my needs. Now you want me to work in terms of blur spot diameter. How do I do that? Is it a simple 1:1 translation? Or do I need to start all over again with experimentation to see what works?

Here’s the answer:

Long version

In standard photographic practice, depth of field (along with the concept of CoC) is considered separately from diffraction. A photographer will understand what CoC means and will speak of a certain CoC as being appropriate for his needs in calculating depth of field. He will give no thought at all to diffraction unless he stops down quite far. If he does stop down quite far, he will know that the CoC is quite small and will correctly see diffraction as a layer of blur added on top of everything. If he is after a sharp image, he will avoid stopping down so far, on account of that diffraction, despite the small CoC. That’s another way of saying that, at large f-numbers, he no longer trusts CoC to be an accurate reflection of image sharpness.

But he did trust CoC when shooting at wide or medium apertures. At such apertures, diffraction is minimal, so the real-world blur (due to the combined effects of defocus and diffraction) is pretty much the same as the CoC. So if he had decided that a CoC of 20 microns is sufficient to produce an image sharp enough for his needs, he made that decision, not when shooting at large f-numbers, but when shooting at low to medium f-numbers. It follows that if a 20 micron CoC will result in a sharp enough image, then so will a blur spot diameter of 20 microns.

If the photographer switches over to thinking in terms of blur spot diameter, he can trust whatever depth of field outputs the app is giving him. He no longer needs to suspend trust at high f-numbers, because diffraction is automatically taken into account.

Of course, there are those extremely rare photographers who, by virtue of the nature of the photography they undertake, have extraordinarily critical needs. Such people will want to undertake a few tests. (In fact, such people are always undertaking tests.)

Short version

Unless he has extraordinarily critical needs, a photographer should use as his target for a suitable blur spot diameter the same value as the CoC that he has been happy with.

© 2013 George Douvos