Lenses and Diffraction Limits

greenspun.com : LUSENET : Large format photography : One Thread

I know from prior threads that some of you know a great deal about this subject. I don't know much. At f:64, what is the greatest resolution one could hope to achieve with a lens like a APO Nikkor 360 f:9? I get the impression that if the lens is decent diffraction pretty much makes them perform similarly when stopped down that far. Second question...does the limitations of diffraction at small stops apply across the board independent of focal length? In other words, is it less of an issue with a long lens like a 400 mm than it would be with a 150, when both are at the same f stop? Please feel free to (correctly) assume I don't know much when you answer and thanks for your advice.

-- Kevin Crisp (KRCrisp@aol.com), April 24, 2001


I believe "do the limitations of diffraction at small stops" would be better grammar. Don't tell me 15 year old.

-- Kevin Crisp (KRCrisp@aol.com), April 24, 2001.

It is probably fair to say that most lenses are diffraction limited at f/64. However, note that poor lenses need to be stopped down further to eliminate various aberrations, while a good lens may need to be stopped down less. In other words, if DOF requirements permit, a better lens will allow you to operate at larger apertures, thereby reducing the effects of diffraction.

The resolution at any f stop can be computed using the Airy disk (the diffraction equivalent of the the circle of confusion) - note that this is an approximation itself but a pretty close one. Resolution is given by 1390/f. This is only an approximation since the Airy disk is the brightest ring but there are larger but dimmer circles that form. Also, the wavelength of exposing light has an effect - red light will yield more diffraction and blue will yield less. Using the approximation though, at f/64, you can resolve approximately 21 lp/mm. So, at f/64, just about any lens should be functional since the ceiling is set more by diffraction than by inherent lens ability.

Diffraction is only a function of f stop. So at a particular f stop, you will have identical diffraction effects regardless of focal length. However, longer lenses have lower DOF and that means you might have to stop down further for DOF requirements which will increese diffraction effects.

Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), April 24, 2001.

This is obviously just my opinion but I think the adverse effects of diffraction are overrated. I've read questions that begin something like "how far can I stop down before diffraction ruins the photograph" where the person seems to think that diffraction is a much greater problem than, in my experience, it really is. I'd much rather get the depth of field right and tolerate the (to me) insignificant effect of diffraction than not stop down enough because I was worried about diffraction and lose necessary depth of field. I use F 64 with 4x5 fairly often. Perhaps I'm not sufficiently critical but the 11x14 prints that I usually make look fine to me.

-- Brian Ellis (bellis60@earthlink.net), April 24, 2001.

Gents, You can get a feel for diffraction by putting a negative with some fairly crisp grain into your enlarger and then, with a grain focusing aid, watch the grain turn to mush as you stop down. Luckily we don't view our prints with a magnifier. As far as camera work is concerned, Brian is right: at low degrees of enlargement, it won't be a problem.

-- Bruce Wehman (bruce.wehman@hs.utc.com), April 24, 2001.

Just to complete the math : at a theoretical maximum resolution of 21 lp/mm, you are basically dealing with an effective circle of confusion on the film of 1/21 mm = 0.05mm for objects in the focal plane. This becomes roughly 0.05mm x 11/4 = 0.13mm on an 11/14 print from 4x5. This is probably not noticeable at normal viewing distances. If, however, you make a 20x24 enlargment, you are looking at 0.24mm, which would definitely be noticeable in areas with fine detail.

-- Richard Ross (ross@hrl.com), April 24, 2001.

Just to chime in again.... Its hard to make generalizations about tradeoffs between diffraction and DOF but.... Its worth keeping in mind that this also depends on the subject itself. Some pictures seem to benefit from the increased focus in near and distant objects to the extent that the degradation from diffraction in the main plane of focus can be lived with (or not noticed since the increased sharpness in other areas seems to compensate or enlargement ratios are not huge enough to matter). Other pictures seem to demand the maximum resolution in the plane of focus, even at the cost of other objects being less sharp. Diffraction in macro work can be particularly nasty since the effective stop is even smaller than the marked stop.

Its worth keeping diffraction in mind for a couple of reasons. DOF is something we are worried about on a much more obvious level. However, since the image on the GG gets dimmer as we stop down, we often do not see the effects of diffraction readily. Also, we are examining the out of focus portions of the image and watching them sharpen up as we stop down - we're not really paying attention to whats happening at the plane of focus. Again, I'm not suggesting wringing hands and paralysis by analysis - its just another variable thats worth keeping in mind since some pictures can be ruined by it.

Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), April 24, 2001.

Ansel Adams voted to call him and his friends Group f/22. But Imogen whacked him with her umbrella till he changed his vote. Ever after he claimed she drove him to diffraction .

-- John Hennessy (northbay@directcon.net), April 24, 2001.

Look at Sexton's books. He frequently used f64 and his prints which I have seen are very, very sharp. So I am with Brian. Diffraction is over rated. Just do some testing with film and paper. That will clear it up for you. Heck. Most people don't align their enlargers properly anyway. James

-- james (James_mickelson@hotmail.com), April 25, 2001.

Check Ron Wisner's site, www.wisner.com, in his Q&A. He spends quite a lot of time on diffraction. As for resolving power, he suggests that all lenses are pretty much equivalent beyond f22, since lenses begin to be diffraction limited at this aperture.

-- neil poulsen (neil.fg@att.net), April 25, 2001.

Moderation questions? read the FAQ