Re: [Leica] Leica's diffraction limited lenses

[Erwin Puts <imxputs@knoware.nl>]

>Per "Leica Fotografie", 8/93. page 27; " The optical performance of the
>lens (280 f4) at full aperture is so remarkable that f4 can actually be
>used as the "optimum" working aperture. Indeed, stopping down does not
>really enhance the performance to any appreciable degree".  Indeed, per a
>LHSA meeting in Kentucky, as I recall, the folks (Lothar Kolsch?) from
>Leica Germany announced that the 280 f4 was a "diffraction" limited lens.

Here we talk about two different but related phenomena. Any lens is
theoretically diffraction limited in its resolution characteristics. See my
earlier posting.
Most lenses are computed/constructed in such a way that at full aperture a
number of optical aberrations will degrade the image more or less severely.
Stopping down will lessen these effects and the image will improve. When a
diaphragm of 8 or 11 has been reached the combined effect of optical
aberrations and diffraction will degrade the image even more when stopping
down. The relative importance of both groups of effects will vary of course.
If a lens at max aperture is already at its "optimum" (note optimum is not
equal to maximum) it means that the optical aberrations are exceptionally
well corrected. But stopping down will always improve the performance but
to a less noticeably degree as when we are talking about lesser designs.
Stopping down more will introduce the diffraction effect in its *pure*
form.
When Leica talks about a lens corrected to a certain optimum they are
referring to a lens where the rendition of extremely fine detail is
optimized to the intended use of the lens. This is not necessarily the same
as stating that the lens has the highest possible resolution. And it is
resolution that is most affected by the diffraction effects.
Erwin