The primary (but not the only) reason for a particular maximum stop is often mechanical. Some long lenses let you stop down to f/45. This is because they can. The opening is still large compared to the thickness of the blades.
Regards, Bob... --------------------------- "No man's life, liberty or property is safe while the legislature is in session." -- Mark Twain From: "Lon Williamson" <[EMAIL PROTECTED]> > It may explain why some longer non-Macro Pentax lenses > stop down to f32, though. The M200 f4 and K135mm f2.5 > both do. Anything shorter I own, except for macro lenses, > has a maximum of f22. > > What I understand from the posts so far, and thanks folks, > is that _probably_ a 20mm at f22 is affected more by diffraction > than a telephoto at f22. > > Now I can sleep at night. -Lon > > Bob Blakely wrote: > > This is certainly correct, but the effect is miniscule except for the most > > extreme wide angles. > > > > From: "graywolf" <[EMAIL PROTECTED]> > >>Wouldn't be that simple, would it? > >>The way I interpret it (and I may be wrong, after all I used to think it > > was > >>just aperture), it is more a combination of aperture and angle of view > > that > >>affects diffraction. So you can use f-stop only as an approximation, but > > that > >>ought to be close enough for most work. > >> > >>In other words a 100mm lens at f22 on a 35mm camera has slightly less > >>diffraction than a 100mm lens at f22 on a 6x9 camera does. Of course that > > > > is > > > >>more than offset by the higher magnification needed with the 35mm image. > >> > >>So, go ahead and use f-stop. With your istD the sensor resolution is > > > > probably > > > >>below the diffraction level at the smallest f-stop anyway, in which case > > > > you can > > > >>ignor diffraction. > > > > > >>Steve Desjardins wrote: > >> > >>>Let me see if I have this straight: > >>> > >>>The extent of diffraction is determined by the size of the aperture but > >>>the effect at the film plane also depends on the focal length of the > >>>length. Because both of these are involved, the property I should worry > >>>about in terms of effect on my pictures is f stop, which is the ratio of > >>>these two factors. > >>> > >>>Yes? > > >
