Kittisak Yokthongwattana wrote in article ...

> Yes,my opinion about real-life shooting situations is that there are not
> very often that we can have all objects in the same plane. When we take
> the shot at wide apertures (2.8-5.6) with tele lenses or zoom, the
> subjects in front of and behind the focus point will be off focus. So, I
> was just wondering why a lot of people said a specific lens "A" is so
> sharp edge to edge even at wide open, and lens "B" is very soft at the
> edge wide open, such and such. The point is it's hard to tell whether the
> soft edge really results from the lens or the off focus effect at wide
> open. Am I right? :)

Yes, if you shoot only images in which part of the subject is in the center,
and in focus, and the rest of the subject (everything within the photographic
frame, BTW, may be regarded as "subject"...) is out of focus (often the case
with AF-"assisted" images...;-). But many photographs will have edge and
corner features in nearly the same plane as the center feature, and a poor
lens used nearly wide-open may never show the image edges and corners
sharply rendered, but a good lens can...

> The reason I had this stupid question in mind was because I used to shoot
> some pictures with Nikon 70-210 f/4-5.6. The images taken at 210/5.6 is
> very sharp at the center, main object, and out of focus at the edge (as I
> focus at the center). Now I own the Nikon 80-200/2.8, the images taken at
> 200/5.6 were not that different from the 70-210. Almost the same center
> sharpness, and out of focus at the edge.

If you shoot a detailed horizon line with the camera tilted so that the
horizon extends from one frame corner to the diagonally opposite
corner, and if you use good shooting technique, the 80-200mm f2.8
Nikkor should show excellent detail to the corners at f5.6 (and even
at f2.8..., unless it is defective) - the 70-210mm f4-5.6 may not.
Hope This Helps
David Ruether - http://www.fcinet.com/ruether