In article , sleicher@cheme.washington.edu says... (much deleted)
>Both of us agreed that the viewfinder results confirmed the film >observation results I had made earlier: at 200 mm, the image of the >Sigma was marginally better than the Nikon. (...)

I will state flatly, based on much experience checking lenses:
lens sharpness CANNOT be judged from a viewfinder image! Lenses
that look marginal in a v.f. can test very well, and lenses that
look great in the finder can be quite poor. A GOOD v.f. can be
used to check for gross misalignments and field curvature in
lenses, but not for sharpness or contrast (unless the lens is really terrible). Also, you do not tell us at what distance you compared
the two lenses. At 200mm at wider apertures and near minimum focus,
the (non-D) 80-200mm f2.8 AF Nikkor is not a great performer.
At mid to long distances, the lens is superb center-to-corner
wide open. (This is also why chart testing is not very useful,
IM(NS)HO - many lenses change performance with distance.)
Hope this helps.