>Well, I'm pretty sure that *IS* how PP get their ratings. Also, look at
>their field curvature plots. They will tell you just how sharp the
>corners can be if you have the center in focus. Most people want the
>center in focus. Except for landscapes, it's rare that everything in
>a picture would be within the "depth of field" anyway, so corner
>sharpness is not a big deal for many people.

Hi-- There is field curvature (with overall sharpness possible on the
curved "surface of focus"), and then there is relative inability to image
well on some parts of that surface - different issues, I think. Also, I
think it is more common for a focused texture or some other focused subject parts to run near the corners of a photo than not (not just in w/a photos
where there is much DOF, but in long lens photos where a line of focused
ground, water surface texture, etc. often runs across the frame or near
a corner). I do agree that corner sharpness is not a big deal for many
people, but I think that indicates a certain lack of standards (kinda like saying, "If the paint is good on most of my new car, I guess I don't care
if there is no paint toward the front and back of the car - who pays
attention to the ends of the car, anyway?"). Or, to put it another way,
I consider it somewhat unsophisticated to view only part of a photograph
as important, i.e., "the subject" - to me, everything within the frame
edge is part of the photograph and needs to make sense as part of the
image. A lens that is unable to image the subject well over the whole
frame is a very limiting lens, since its characteristics are clearly
imposed on almost all images made with it.
David Ruether