Diffraction limiting is a technical term. Diffraction is one of the things that cannot be mitigated by the wonders of modern glass or lens design. Diffraction is solely determined by the physical size of the aperture and it's deleterious effects relative to the image at the focal plane is solely determined by the f/stop. Diffraction limited means that the effects of diffraction (which cannot be mitigated) are the major factor effecting sharpness. If not Diffraction limited, then the major factor effecting the sharpness of a lens is in the lens design or in the glass itself. A poor lens will be diffraction limited at between f/11 and above. An average lens will be diffraction limited at between f/8 and f/11. A good lens will be diffraction limited at between f/5.6 and f/8. An excellent lens will be diffraction limited at f/5.6 or larger (aperture). All astronomical telescopes worth a damn are diffraction limited.
The effect of diffraction limiting is the same for all focal lengths at any given stop. Regards, Bob... From: "Lon Williamson" <[EMAIL PROTECTED]> > Read any book about the technical aspects > of photography, and you'll run across > diffraction limiting, which is usually > supposed to kick in at an F stop of 16 or > 22 and be noticable by 32. > > Now compare the absolute size of, say, > a 20mm prime stopped down to f22 and a 100mm > prime at f22. Your eyeballs tell you the 20mm > aperature hole is a lot smaller than the 100 hole, > but the 100 hole is farther from the film plane. > > Question: Is it the absolute diameter of the aperature, > or some formula taking into account diameter _and_ > distance from the film that causes diffraction limiting? > > In other words, does diffraction limiting affect a 20mm > lens set at f22 _MORE_ than a 100mm set at f22? I asked > a physicist friend about this one time, and he started > talking like a politician. In other words, to put it > bluntly, he bullshitted me rather than admiting he didn't know > and couldn't calculate a reasoned answer.
