Alfred Molon <alfredDELETE_molon DeleteThis @yahoo.com> wrote:
: At least that is the case with the HTMC coated filters from Hama I'm
: using. Or are there high quality coated polariser filters which do not
: take away so much light?
All pola filters will have some degree of light transmittion blocking. If
you think of the pola filter as a slat type window blind with the slats
aligned with the direction of the light path. The slats do have thickness
and thus block a portion of the light. The slats in the Pola may be
microscopicly thin but they still block a portion of the light. And this
is only increased when we require a circular Pola as the twist after the
pola that makes it a circ pola would also reduce direct light
transmittion. Then since to work tha pola is blocking all the light waves
that are undulating off axis to the filter. If you figure that up to 50%
of the light will be naturally off axis, this alone would account for
reducing light transmittion by aproximately half.I would also suspect
that even the anti reflective coatings would add some light blocking
factors. The fact that all these possible light reduction factors only
reduce the light by 2 stops is a wonder.
I would suspect that there may be some ways to make a less dense pola that
would reduce the light less, but it would likely be much less effective in
rejecting off axis light (what a pola is supposed to do). And such less
effective polarizers may also be less optically clear and thus cause
slight optical abberations or inconsistancies in density across the field
of view.
Personally I plan to stick with the best quality pola I can afford and
just adjust for the 2 stop reduction. I generally don't need to use a pola
in low light conditions, anyway.
Randy
==========
Randy Berbaum
Champaign, IL
>> Stay informed about: Why do polariser filters take away two stops of light? 
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