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f-stop --> diffraction vs DOF

Keith Baker , Nov 10, 1999; 07:27 a.m.

I was following a recent thread and was reading that the sharpest image with 35mm equipment is about f-8. If you go to f16 or f22 you lose sharpness due to diffraction. I was also noting that the large format folks usually go to f22-f45. Are the LF folks losing sharpness to gain DOF? Does LF give such a fine neg that the DOF gain outways the sharpness loss? Thanks for the comments in advance. - Keith

Responses


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Howard Slavitt , Nov 10, 1999; 08:38 a.m.

Diffraction does not have a significant effect in large format until you reach about f22 (sharpest fstop without diffraction changes based on format). Also large format lenses are designed to be sharpest at about f22. If they were shot at f5.6 or f8, they would give much less desirable results. Also, the loss of sharpness due to diffraction is often overemphasized. It's an issue to be aware of, but if you need the depth of field you're much better off taking a diffraction tradeoff than having insufficient depth of field. If you're wondering about how large format can be so much sharper than 35 mm, it's because of image size

Dhananjay N , Nov 10, 1999; 08:45 a.m.

Short answer, no. The basic idea is that when you go to f/16 or f/22 with a 35mm camera, what you end up with is very close to a pinhole which is diffraction limited. In other words, you're not using a lot of the glass you paid for. However, when you move up to larger formats, you're typically using longer lenses. So, while 50mm is normal on a 35mm camera, 150mm is normal on a 4x5. f/22 on a longer focal length is not really all that small, its still a fairly large aperture. So you can stop down a little further with larger formats. Howeever, you will hit thesee limits with those formats too, its just that you hit it later. Larger formats also have the advantage of not being enlarged as much. This means that for a similar resolution on the print, you have lower demands for resolution on film with larger formats. DJ

Alan Gibson , Nov 10, 1999; 08:50 a.m.

Very roughly, for a given angle of view, and a given final print size, the diffraction on 35mm at f/8 is the same as on 120 at f/16, and 5x4 at f/32. The depth of field is also the same. So the answer is: No, the LF folks are not losing sharpness to gain DOF.

Bill Mitchell , Nov 10, 1999; 09:39 a.m.

Diffraction effect depends strictly on the size and shape of the hole (the aperture) that the light waves must pass through. The smaller the hole, the more diffraction. Since lenses for bigger cameras are larger (for the same f stop), there is less diffraction for them. Diffraction starts to be significant for 50mm lenses at moderate apertures, but only becomes a problem with image sharpness for most lenses by f:22, which is the reason most stop down only to f:16. For wide angles it's a smaller number, while for telephotos it's larger. The lens for an 8x10 view camera may not have significant diffraction effect until it's stopped down to f:64, or even more. Also, since the amount of enlargement will be less, the amount of fuzzyness of the image which will be tolerated can be a lot less. Edward Weston sometimes needed more depth of field than his minimum aperture would allow, and punched even smaller holes in black paper which he used as the aperture. Since he printed strictly by contact, his prints appear to be razor sharp. Generally speaking, unless you are a lens designer, diffraction effects are something to be aware of, and properly ignored. It is a subject often overemphasized by some tech-speak photographers with insufficient practical experience.

triblett Lungre-Thurd , Nov 10, 1999; 10:26 a.m.

great answer Bill, thanks

Larry H. - Atlanta, GA , Nov 10, 1999; 10:38 a.m.

I'm trying to clarify what Bill just said. Is diffraction a function of the relative aperture (i.e., f/16), the absolute aperture (i.e., 50mm/16 = ~3mm; 100mm/32 = ~3mm), or both? Does it really depend on format size (other than enlargeability, obviously), or just on the fact that larger formats use longer lenses for the same field of view?

Alan Gibson , Nov 10, 1999; 10:53 a.m.

Diffraction on the film is a function of the relative aperture, and that is all. Diffraction on the print also depends on the degree of enlargement. Larger formats are enlarged less.

Dan Nguyen , Nov 10, 1999; 11:24 a.m.

Keith,

You've gotten some good answers. Just to be sure, here is yet another from the archive: http://photo.net/bboard/q-and-a-fetch-msg.tcl?msg_id=000WVM

Also, my Nikkor 24-120 is sharpest with f8 at the short end, but f16 at the long end.

Hope this helps.

Mark Ci , Nov 10, 1999; 11:37 a.m.

Bill's post is misinformation, and misinformation that's been posted here before recently. Please stop it.

Diffraction effects on the film depend on focal ratio. At a given f-stop it's no worse for wide angles, and it's no better for telephotos. It's no better for larger formats on the film, either. The only way it's better is if you assume less enlargement when printing the larger format.


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