In article <4hnjak$ku6@cello.hpl.hp.com>, jacobson@cello.hpl.hp.com says...
>In article <4hnaar$9gv@newsstand.cit.cornell.edu>,
>Bob Neuman wrote:
>>The 100mm f2.8 [Vivitar] Series I macro (also, in the early version,
>>a "Kiron",
>>I think) was also excellent, and did not have the fatal flaw of the
>>otherwise excellent 90mm: flare when used at non-macro distances.
>>I have tried several samples of the Series I 90mm, and the flare
>>(even with VERY deep shades) was insufferable. I have not been fond
>>of the other Series I's that I have tried (28-90, 28-105, some samples
>>of the 70-210 versions [some were good], and the 100-500)

>David (d_ruether@hotmail.com),
>You seem to be an expert at evaluating lenses without all the formal
>testing most of us use. So you is it that you go about testing a lens
>for flare?
> -- David Jacobson

In the case of the Vivitar 90mm Series I, the flare problem was
discovered in use - too many photos had light spill across them.
It is hard to see the diffuse spill-light in the viewfinder -
though it is quite easy to spot it in a slide when half the area
of the slide lacks black (brilliance) from a white sky light or
other large out-of-frame bright light source spilling to where
it ought not to be. (We have a lot of dark glens around here
which do sorely test lens flare characteristics!) The cause
of the problem in the 90mm Vivitar seems to track back to
a shiny, fairly long truncated conical lens element spacer
at the rear of the lens. To check lenses for flare, I focus
on the bare lightbulb in my darkroom from the next room at night
(with the lights out in the next room). I move until the door frame
obscures the bulb, then move to bring it into view (at different parts
of the frame - and at points out of the frame), watching for the spill
light (but moving my eye position at the finder around to isolate the
effects of the viewfinder optics). Using the DOF button at different
apertures also helps to isolate the different effects (lens flare,
diaphram reflections, and viewfinder flare) that may exist
simultaneously. This is usually not a definitive
test, but can help identify some
lens problems.
Hope This Helps