In article <4gucvl$ov@shellx.best.com>, hal9000@shellx.best.com says...

>Can anyone tell me whether noticeable diffraction occurs at much
>slower F stops like F/22 on a 50mm lense? I had always had the
>presumption that if you can try to go for the maximum f stop for
>sharpest picture and best depth of field. I have talked to many
>others on the net who say they'll just place the camera on a tripod
>and leave the shutter open for 4 seconds just to get the highest f
>stop. Is this bad to do or what?
>hal9000@best.com

Ah, so you missed the very recent discussion about this, huh...?
Uh, yes it's bad ;-). Here is a rewrite of what I have posted before:

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
otherwise good wide-angles require smaller apertures to peak in
the corners, and some particularly good lenses peak at slightly
wider apertures). Apertures smaller than f8 will show detectably
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
generally 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 also generally gets less
sharp as the aperture gets wider). The minimum sharp f/stop is
not different for different focal-lengths for the same
format, like a 28mm lens and a 105mm lens for 35mm. 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). The minimum f/stop does change with
changes in format, i.e., it is different for a 100mm lens used for
35mm format than for a 100mm lens used for 6x7 format. Since the
same focal-length is used to cover different angles, more optical
compromises are required (and generally allowed) for the larger
format lens. And, 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. (I do find my limit for 4x5 to be
about f32 rather than the smaller aperture a bit of number-crunching
would indicate, probably since I expect more from 4x5 than 35mm and
want to be able to peer closely at the print made from 4x5 - and I
generally make larger prints from 4x5 negatives than from 35mm. BTW,
one of my lenses has an f128 on it - I tried it just once!)
Hope This Helps