Old timer asks newby-like aperture question

[Bill_1856]

Shooting 4x5 with 162mm Optar, printing no larger than 12x16. At what point of stopping down does the diffraction effect become excessively severe? (Yeah, I know that it's really a matter of opinion, but I'd appreciate hearing what your experience is.)

[Dan Fromm]

Hmm. Going from 4x5 to 12x16 requires a little more than 3x enlargement. So you'll need a little more than 24 lp/mm in the negative to get 8 lp/mm in the print. Say 30 lp/mm.

The rule of thumb is that the diffraction limits resolution to 1500 (some use 1600)/(f/#).
If you want 30 lp/mm, the largest f/# you can use is 1500/30 = 50.

[Brian Ellis]

I've never found diffraction to be a significant (or even noticeable upon close study) problem when printing from a 4x5 negative, especially not with prints like yours of only about 3x magnification. If you were making prints in the 30x40 range it might be a problem but almost certainly not at 12x16. I'd use whatever aperture gives you the needed depth of field and not worry about diffraction.

[paulr]

the question is, at what aperture does degradation from diffraction outweigh the improvements in aberrations that you get from stopping down. this always depends on the lens. in general the better the lens, the wider the optimum aperture.

[Jim MacKenzie]

The best way to know for certain is to conduct your own tests. Also, the larger you enlarge, the more degradation you will have.

Generally, large format lenses (at least for 4x5) are going to be near their best at about f/16 to f/22. Below that you start to get diffraction-related sharpness loss. Of course, whether this matters depends largely upon whether the increase in depth of field from the smaller aperture is needed in your image, and how big the final image is going to be.

Also, it depends on how sharp the original lens is. If the lens is only mediocre to begin with, f/45 might not actually cause any loss in sharpness. Conversely, with an exceptional lens you might notice diffraction-related sharpness loss at even f/16.

This is why testing is useful. It will help you to figure out how far down you can go and still be happy. As a bonus, you can simultaneously test to see how wide you can go and still be happy, which can be useful in dimmer light, particularly with slower films. Large format lenses are often better at wide apertures than most LF shooters think they are.

[Leonard Evens]

I agree with Dan's calculation.