nat spake thus:

Try rec.photo.equipment.[plug in your format here].

(Two of the formats are .large-format and .35mm.)

  Probably not quite on-topic here but what is your
question?
  Briefly, a circular polarizer is a linear polar filter
with the addition of a "retarder" plate consisting of a
bi-refringent l/4 wave plate. The plate essentially undoes
the polarizing on one side of the filter. This kind of
filter works like a standard linear polarizer but delivers
non-polarized light to whatever follows it. In photography
circular polarizers are necessary for many cameras with
through the lens exposure metering because the metering
system may be sensitive to the polarity of the light coming
in. Most makers of polarizing filters offer both linear and
circular models.
  See
http://www.fas.harvard.edu/~scdiroff/lds/LightOptics/CircularPolarization/Circul
arPolarization.html
  for a pretty good explanation of how they work.

  If you have a polarizer and don't know which type it is,
hold it up to a mirror. A circular polarizer will look gray
on one side and black on the other, a linear polarizer will
look gray on both sides.