Quote Originally Posted by Doremus Scudder View Post
Degradation due to diffraction is a direct physical result of the size of the aperture. It has nothing to do with lens design.

Keep in mind, however, that even with rather stringent standards for the circle of confusion in the print, the effects of diffraction from a 4x5 negative (printed fairly full-frame) shot at f/45 will not be visible to the critical viewer until the prints are significantly larger than 16x20 inches. Even a 20x24 will not exhibit apparent degradation from diffraction unless the viewer is less than 18 inches or so from the print, much closer than "normal" viewing distance.

Upshot: we can safely use very small apertures on 4x5 and still print fairly large without significant image degradation from diffraction. An 11x14 inch print from a negative shot at f/64 still has a diffraction-limited CoC of about 0.066mm, which is below the resolution powers of the eye at normal viewing distances.

To answer the original question: I like f/32 on 4x5 a lot. It's my "go to" stop if I have any doubts about DoF issues and seems to be one I end up using most anyway. I measure focus spread and set aperture for optimum based on a CoC of 0.066mm. I don't have any qualms, however, about using f/64 if needed for DoF. I just make a note not to print that image too large.

Given that most lenses designed for 4x5 use have their optimum aperture at f/22, and that diffraction degradation is not objectionable (for me at least) till f/45 or greater, the aperture range to use is a no-brainer. The only exception is for those who want out-of-focus areas in their prints and must use larger apertures, or even shoot wide-open. Lens aberrations at larger apertures are much more of a concern than diffraction however...

Best,

Doremus
All absolutely true. In the old days when using very small apertures I would make a note to myself that I wouldn't print that particular image as large as I might otherwise. Now, diffraction is especially not much of an issue when printing digitally in whatever form as a slight touch increase in sharpening will effectively nullify visually any effect from diffraction.