Curiosity testing for diffraction limitation -- very general

[Ulophot]

I included "very general" in the title, because I am not worried about numbers here. I have two lenses, a Komura Commercial 210 f/6.3 and Nikkor-S 135 f.5.6, which stop down to 45 and 64, respectively. Since my principal interest is B&W portraiture and I shoot only HP5 and print only to 11x14, both of these lenses is more than adequate for this purpose. I already know this and am perfectly happy with both lenses.

I am posting this in case someone sees something I am leaving out in my considerations, below.

I do shoot other subjects, and recently was shooting some flower and leaf still life scenes in the garden fairly close up, requiring small apertures even after movements. It piqued my curiosity about the lenses' performance at these f/32-and-smaller apertures, where I am aware that diffraction limitation enters in. Thus, just for curiosity's sake, I wish to run a simple comparison.

My idea is tape some black nylon screen material tightly against the back wall of our fairly smooth, light-colored stucco house and make images at f/16 (which is probably near optimum sharpness for these lenses) /32, and /45 (plus /64 with the 135) from 5-6 feet away.

I know that various lenses are optimized for smaller or greater distances; again, this is very general, and unless one of the lenses really starts losing sharpness at 2-3 feet that could affect such close-ups as I mentioned, which I find highly unlikely, this test, with screening pretty much filling the frame, should offer reasonable comparison of sharpness at the apertures selected.

I know that the enlarger must also be aligned, that it, too, has a lens, and that the photographic process has more variables than a barrel of monkeys. Again, I'm not going for numbers, just planning to poke around, so to speak, to see if the limitation will show up at all, and if so, how much, within my general parameters.

[Bob Salomon]

Those lenses should be optimized for 1:20 which means they should perform well down to 1:10. Closer their performance should fall off. They should also be diffraction limited at f22.
So you could have 2 problems with your tests should you be shooting closer then 1:10 and at apertures smaller then 22.

[Eric Woodbury]

Be sure to read this and the article mentioned near the bottom by Dr Hansma.

https://www.largeformatphotography.info/fstop.html

It is said that optimum f/# for LF lenses, and maybe others that are affordable by common man, is two stops down from wide open. This reduces spherical aberrations certainly. Also, it is about the point where aberrations and diffusion are equal. As you continue to stop-down, the aberrations fade and the diffusion comes on stronger.
Sometimes a little aberration or diffusion is better than 'out of focus', but sometimes not. This is where your experience is needed.

Don't worry about it too much. There are plenty of other things to occupy your mind.

[mdarnton]

If I were doing it I'd shoot newspaper, and I'd shoot at a slight angle to minimize the effect of focus shift (since you'd always be able to find somewhere on the tilted paper that was in focus, focus is out of the problem.) I say newspaper because that kind of high contrast and variable subject can tell you more than I think a screen would.

The best subject is probably the good old resolution test chart, but then you have to be more aware of focus shift and error.

[Dan Fromm]

I wish there were a hard-and-fast rule but it really depends on how much you want to enlarge.

[Jeff Conrad]

Diffraction depends on the effective f-number, so it gets a lot worse in close-up work. I discuss diffraction in detail under “Diffraction” in https://www.largeformatphotography.i...DoFinDepth.pdf. Alas, I didn’t give any examples for close-up photography.

[Ulophot]

Thank you, Bob, and thanks to all.

[Alan Klein]

Since you shoot portraits, shouldn't DOF be an important criteria? Normally, you'd want DOF to extend from the tip of the nose to the back of the head. If you use too open of apertures, you start getting out of focus noses and ears. If you go too small, you start bringing into focus backgrounds. In any case, I don't see how you get into diffraction with DOF used for portraits.

[Alan Klein]

Diffraction may be more of an issue with landscapes where you're looking for maximum DOF's.

[Bob Salomon]

Since you shoot portraits, shouldn't DOF be an important criteria? Normally, you'd want DOF to extend from the tip of the nose to the back of the head. If you use too open of apertures, you start getting out of focus noses and ears. If you go too small, you start bringing into focus backgrounds. In any case, I don't see how you get into diffraction with DOF used for portraits.

We were taught tip of nose to base of ear.