In article <96052.092649U15310@uicvm.uic.edu>, U15310@uicvm.uic.edu says...

>I used to know the answer to this but seem to have forgotten...
---Probably because there is no one answer......
>Can anyone tell me:
>1) For a given format, say 35mm, what is the minimum f/stop before >diffraction results in loss of sharpness.
---For any perfect optic (any format), any aperture smaller than ---wide-open would show loss of resolution due to diffraction.
---BUT, in the real world of compromised optics, virtually all
---good 35mm-format lenses peak in sharpness around f8 (some good ---super-wides require smaller apertures to peak in the corners, and
---some particularly good lenses peak at slightly wider apertures).
---Apertures smaller than f8 will show reduced sharpness due to ---diffraction, but noticeable lack of sharpness doesn't begin until
---just after about f16 (f22 is useable, but is a definite compromise
---between DOF and sharpness). BTW, wider than about f8, undercorrected
---optical problems swamp the potentially greater sharpness that the
---diffraction limit would allow, and the divergence between the
---sharpness of the lens and the diffraction limit gets greater the
---wider the aperture (and the lens generally gets less sharp as
---the aperture gets wider).
>2) Is the answer to the above contingent upon the focal length of the >lens, again, staying with the same 35mm format. That is, is the >minimum sharp f/stop different for a 28mm lens than for a 105mm lens?
---No. But different lens design types (related to angle-of-view)
---have different compromises which result in variations in performance
---(generally most visible at the edges and corners of the frame, and
---sometimes related to focus distance).
>3) Does the minimum f/stop change with changes in format, i.e. is it
>different for a 100mm lens used for 35mm format than for a 100mm lens >used for 6x7 format?
---Yes. Since the same focal-length is used to cover different angles,
---more compromises are required (and generally allowed) for the larger ---format. Since magnification of the film is generally less with the
---larger format, noticeable sharpness degradation due to diffraction
---doesn't set in until smaller apertures are used than would be
---acceptable with 35mm. Therefore, f22 would be OK with medium-format,
---f32 with 4x5, etc.
---Hope This Helps