Jedi Lee wrote in message <19980217051200.AAA21880@ladder03.news.aol.com>...

>Will someone give some solid information to validate or invalidate the
>statement that F 5.6 is the sweet spot of a lens. I thought that it was
>proprietary to the lens manufacturor, and what type of lense it was.
>I would really like to know if F5.6 is the sweet or not.

Optimum aperture for a given lens is determined by many things, but
IN GENERAL, good-quality zoom lenses intended for very small formats
like 1/4-1/2" CCDs may often peak around f5.6. The difference between
performance at f5.6 and adjacent whole stops (f4 and f8) may be very
difficult to see, except maybe in the corners at the short end of the
zoom range. Adding a wide or tele adapter will most likely result in
moving the optimum aperture to a smaller stop. Prime lenses of good
quality (or almost any multi-kilobuck high-end lens) will probably
optimize at wider stops. Good lenses designed for larger formats
will generally reach their optimum apertures at smaller stops
(f8 for 35mm, f11 for medium format, f16-22 for 4x5, etc.),
If you are worried about what stops perform best (a
reasonable concern, I think...), then try carefully
focusing a detailed scene (best a distant one, so that
DOF is less an issue), and run through the f-stop range
on the lens and judge for yourself the results. BTW, the
optimum--aperture effect is caused by the crossing of two
performance curves: the improving performance with stopping
down caused by having most lens aberations reduced with simple
reduction of the area of the lens through which light passes to
the (best) central area; and the worsening performance with
stopping down caused by increasing diffraction effects
around the aperture with smaller stops, effects
from which no lens escapes.

--
David Ruether
http://www.fcinet.com/ruether
ruether@fcinet.com