In article <32ECF9B7.29B1@direct.ca>, cyuill@direct.ca says...
{most of very nice post deleted, to address one point...]
>The article contained photos of the filters taken with polarized light, which
>showed stress marks created by the manufacturing. Not surprisingly the Cokin >filters showed stress marks over most of the filter, indicating that the >plastic had been stretched during its formation. The Nikon showed uneven >stress marks, whereas the Hoya's marks were almost perfectly concentric, which >supposedly indicated that the Hoya filters were made MORE carefully than the >Nikons!?!!!??!!! (I think that this indicated that the Hoyas were more >parallel.)

I would not come to that conclusion... (much as I like Hoya filters...;-).
When polarized, the plastic filter is likely to show stresses no matter
how flat; Hoya glass is held in the rim by a retaining ring that is uniform
around the circumference (and perhaps holds the glass too tightly, Nikon
might say...;-), resulting in uniform stress, but potential problems
in extreme temperature conditions; Nikkor filter glass is often retained
by a fine wire that applies pressure to the glass at only a couple of
points (stressing the glass, but with no ill effect), allowing the glass
to move in the frame when necessary. I don't think Nikkor glass filters
are any less flat than Hoya filters....
Hope This Helps