Basically I shoot f/22 because someone told me that's a good place to start when I took up 4x5 LF photography during Covid for the first time. I'll switch to f/32 if I think the DOF requires that based on DOF charts I use. I don't worry much if I'm using 90mm or 75mm with 4x5 because of the greater DOF anyway with those lenses. Mainly its when I shoot 150mm or 300mm with 4x5 that I wonder. Considering your guidelines, I probably shouldn't worry too much as I don't print that large anyway. The "fun" for me with LF photography is the technique and handling which can get frustrating at times as well. I really should send out a favorite for printing real large like 32" x 40".
The suggestion to use f/22 or f/16 on most LF lenses is because that is the sweet spot where stopping down has ameliorated lens aberrations and the effect of diffraction is still small. However, that only applies to the plane of sharp focus itself (which is what the lens manufacturers are basing their tests and specifications on). Once depth of field comes into play, then the extremes of the scene that one desires to have in sharp focus become the operating parameters and the effects of diffraction need to be balanced with acceptable sharpness at those extremes.
The optimum aperture for any given scene, then, depends on the focusing distance between the near and far objects in the scene that one wants to have acceptably sharp in a particular size print at a particular viewing distance. The idea is to have the out-of-focus points at the extremes small enough in the final print so that the eye resolves them as points.
The best way to find the optimum aperture for any given scene is to identify the extremes, i.e., the objects farthest from the plane of sharp focus on either side, and find the best compromise between depth of field and diffraction.
One can do this visually: Say one makes 16x20 prints from 4x5 negatives. That's a 4x enlargement without cropping. So, one could simply get a 4x loupe and spend time examining the extremes under the loupe at different taking apertures until one found the best one. Or, one can use a method where one measures focus spread between the two extremes and then selects the optimum f-stop based on a table that takes all the parameters (depth of field, diffraction, degree of enlargement, viewing distance) into account. Or one can use depth-of-field tables for the focal-length lens used after measuring the distance from the camera of the near and far extremes.
Any of these work; simply using f/22 for everything won't. In my experience with real life subjects and the kind of depth of field I want, I end up using f/32 more than any other stop on 4x5. My 16x20 prints made from negatives at that aperture show no noticeable sign of diffraction. (Note: for 8x10 and a 16x20 print, this would be good old f/64, just like the group - that's for a 2x enlargement of an 8x10 negative too. For contact prints from 8x10 negatives, one could easily use f/90 or smaller. I'm sure Edward Weston did. kfed1984, take notice).
And, as Drew astutely points out above, judicious use of camera movements can help optimize the aperture as well by reducing the necessary depth of field.
I'll use f/45 if needed, making a note that a smaller print will be optimum. Still, I have never run across a case where stopping down for depth of field wasn't better for the final image than using a larger aperture for less diffraction degradation and the resulting reduced depth of field.
My point being: there are better ways of finding the optimum aperture for a scene than just using f/22 or whatever. And, that depth of field trumps diffraction almost every time. Furthermore, diffraction degradation really only becomes apparent at very, very small apertures and/or high degrees of enlargements. If you're making 11x14 and 16x20 prints from 4x5 negatives, you shouldn't be worrying about f/32.
Best,
Doremus
