> Don't all lenses share that? I guess I'm confused as to why you > specified the 77/1.8 in that way.
Dan, "Diffraction limited" means that diffraction is the main aberration--"masking" all others. Since diffraction _can't_ be done away with, when diffraction is the dominant aberration it means that the performance of the lens basically can't get any better. With most (good) lenses, there is an "optimum" f-stop...one at which the principal aberrations have been minimized but diffraction has not yet begun to degrade the image quality too badly. Since diffraction has less of an effect at wider apertures, the optimum aperture of a lens is the _widest_ aperture at which the basic aberrations (spherical, chromatic, etc.) are brought under control. A _lens_ is said to be "diffraction limited" when it is diffraction limited at its widest aperture. Strictly speaking, there are no diffraction limited camera lenses. Maybe a few enlarging lenses come close. And there are a few short teles (like the Leica 90mm Elmarit-R) that are diffraction limited at maybe one and a half stops down, which is damned good. Hope this helps, --Mike
