In article <505eck$7o5@lectura.CS.Arizona.EDU>, bmtong@CS.Arizona.EDU says...
>In article <3225A59B.4D18@mail.bakerbooks.com>,
>Daniel Knight wrote:
>> Sigma 21-35 (discontinued)
>
>This lens is one of the sharpest lenses I have owned, and has very
>little distortion. Unfortunately, this lens is very flare-prone.
>I believe flare is the real enemy of wide angles, not sharpness.
>For this reason I would suggest approaching any wide angle zoom
>with extreme caution.

Hmmm, in my experience, VERY few wide-angles are what I would call
sharp, unless stopped down to f11-16, and even then, many are not
what I would call sharp (including the Sigma 21-35mm that I tried
[and even some Nikkors, like the 18mm f4, and the pre-f2.8 compact
20's]). The difference in opinion may be caused by different
sensitivity to edge/corner performance. To me, a lens that is
unsharp anywhere in the frame (due to optical problems, rather
than focus selected) is unsharp (it does not cover the format,
and is therefore unacceptable for high-quality image-making
[though it may be useful for special purposes]).
Hope This Helps