How is use of f128 possible/practical?

[sanking]

Robert, I have an ancient Wollensak f10 process lens of 19" (protar 8 elements 2 groups type) that easily covers the 12X20. I have stopped it down to f90 1/2 often but that seems to be a practical limit for me. Beyond that and I consider even the contact prints too soft from diffraction.

I have observed this as well. In theory stopping down to f/128 should give resolution of 10 lppm, and even at f/256 you should get 5 lppm. Since 5 lppm is considered adequate for contact prints it should be possible to effectively use apertures of f/127 and f/256, if the final goal is a contact print. But as a practical matter I find that stopping down beyond f/90 gives contact prints that to my eye are too soft.

Sandy

[Mark Sampson]

25+ years ago I worked in a custom lab; among other things, I worked on the process camera making copy negatives and such. (No graphic arts work.) One of my tasks was to make b/w internegatives from color trannies; for this we used (ASA 200) Super-XX film. For some reason I can remember that the exposure was 4 seconds at f/45. Smallest f/stop I ever used with any regularity, now or since. Everything else was set between f/11 and f/22, probably to get convienient exposure times. In those days I was ignorant of things like diffraction- but following this thread makes me want to shoot a photo with the lens stopped all the way down, just to see what will happen.

[Alan Davenport]

I'm feeling lazy right now, so I'm not gonna do any math. But:

1. Diffraction is present in every photograph ever made. It rears its ugly head by making (what should be) points on the film become blurry discs. The blur circle of a point in subject space is called, on the film, an airy disc.

2. The size of the airy disc is the same at a given f/stop, regardless of focal length. In other words, the physical size of the diffraction disc on the film will be constant for any lens at f/128. IOW, the diffraction blur -- on the film -- of any lens, anywhere, when at f/128, will be the same size.

3. As long as the airy disc is no larger than the acceptable circle of confusion, diffraction is not a problem.

4. It's generally accepted that the allowable CoC becomes larger with increasing film size. This is both because there is less enlargment required, and because the viewing distance tends to increase with larger prints. A commonly used CoC value for 35mm film is something like 0.03mm, while on 4x5 film a common value is 0.1mm. Larger films can tolerate more blur because they don't have to be enlarged as much.

Therefore: at some point on the scale of increasing film size, there should be an acceptable circle of confusion which is larger than the airy disc produced at f/128. You can use f/128 on that size film without degradation!

[Jim Jones]

The one 11x14 enlargement I have that's made from a 5x7 negative shot with a decent lens at f/128 is unpleasantly unsharp. So are many transparancies or enlargements from 35mm shot at f/16 or f/22. Much depends on the subject, though. Images with severe diffraction limiting might be satisfactory when the subjects are clouds or nudes.