In article <5qtji4$kna@news.sdd.hp.com>, patrick@sdd.hp.com says...

>Not to pick nits (and keeping in mind that David Reuther has forgotten more
>about lenses than I'll ever know ;-), but...

>In article <5qo9aj$2bn@newsstand.cit.cornell.edu>, d_ruether@hotmail.com (Neuman -
>Ruether) writes:
>|> Never heard the above ones, but I do often hear:
>|> 4. Lenses are sharpest about two stops down from wide open.
[...]

>If (hypothetically) somebody handed me a lens about which I knew nothing,
>and I had to make a shot that required maximum sharpness with it, I'd go
>about 2 stops down (actually, I'd bracket in the range of 1-3 stops down
>unles I knew something about the lens which made me do otherwise; See below)
>Though seldom exactly true, the "two-stop" rule is typically accurate
>enough to serve as a worthwhile rule of thumb if lacking better information.
>
>The only "killer" exceptions I can think of are lenses which are diffraction-
>limited within 2 stops of their maximum aperture, and lenses which are
>heavily optimized towards operation at their maximum aperture, such that
>they may actually exhibit worse aberrations when stopped down; The Noct-
>Nikkor comes to mind... Presumably the photographer would be aware of it
>up-front if they were dealing with either of these cases, though.
>
> -- Patrick

I don't want to weigh down a light-'n'-fun thread, but, well..., uh,
there are so very many exceptions to the "2-stops down" rule of thumb,
that it is nearly worthless - especially in view of another that works
almost all the time (including for the Noct Nikkor...;-), "for 35mm-format
lenses, best sharpness in the center occurs almost always around f8"...
Hope This Helps
(David Ruether - http://www.fcinet.com/ruether )