In article <4c0ri5$hdp@kodak.rdcs.Kodak.COM>, gindele@eastman says...
>I use a Sigma 21-35 zoom lens with a Canon EOS A2. (...)
>For these situations, the most important feature of the lens is low >geometric distortion. This is one performance specification which >affects final quality at any optical magnification. It is also one >aspect of a lens that can be judged (qualitatively) by looking through >the viewfinder. (rest deleted)

Alas, with most viewfinders, distortion cannot be evaluated using the
viewfinder (though a grid screen helps), since most SLR finders have
themselves considerable pincushion distortion. If you want to check
your VF, put on a prime short tele or macro lens (lenses least likely
to have linear distortion) and line up an easily seen straight line
(distant horizon of a lake, road center line on a straight stretch,
edge of window [with the thin bright exterior just showing at the
frame edge, etc.]) with the edge of the frame. Unless you have
an old SLR, or a Nikon F series camera (and maybe a few others),
you will see the curvature introduced by your VF.
Hope this helps.