On Monday, December 16, 2002, at 01:59 PM, Mike Johnston wrote:
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
Yes, it does. Thanks,
Dan Scott