In article <4l27mg$nmp@newsbf02.news.aol.com>, macbethdiv@aol.com says...

>The rule of thumb I have always heard and used is a lens is sharpest >two to three stops closed from its widest aperture.

I sure would like to kill off that rule of thumb, since it is so often
wrong, and substitute an easier one that is more often true:

"For the 35mm format, most good lenses
are sharpest in the center around f8."

The earlier one probably came about when almost all lenses were slow
and about the same speed, and their optimum apertures were at about
two stops down from the (slow and similar) maximum apertures - now that we have a lot of f1.4, f2, and f2.8 lenses around, the old rule doesn't hold up very well.
Hope This Helps