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[OM] Any advice on filters?

Subject: [OM] Any advice on filters?
From: usher99@xxxxxxx
Date: Tue, 4 Mar 2014 19:14:50 -0500 (EST)
CN writes:
>>But I do agree that I think the circular polarizer is not
>>quite as effective.  The only circular polarizer I have for digital 
use
>>is a 72mm B+W Kaesemann which I bought on ebay from a Chinese seller 
for
>>not very much less than I could have gotten it stateside.  When I 
first
>>got it I was concerned that (despite it's wonderful construction and
>>machining) it might be a fake since the polarization effect on sky 
and
>>clouds wasn't as strong as I suspected.

Harumph, nary a listee read my post, not even Chuck this time.  I 
suppose that is reasonable given the dismal
senorita count.

  The only real difference with the circular polarizer is the 1/4 wave 
plate--typically designed for the middle of the
visible spectrum--though this does vary.  The 1/4 wave plate will NOT 
make perfectly circularly polarized light for all wavelenghs
so in a highly polarized sky there is a slight spectral shift to the 
warm side--PERHAPS that is the perceived difference.
I have never seen a formal study of this though and it is a working 
hypothesis.  The polarizing foils used have different efficiencies at 
various wavelengths as well but that is true for all the pols.

Chuck is spot on about the AA filters and possible interaction with 
linear pols--birefrigent materials are used in the AA--one slab, then 
1/4 wave retarder, then second slab so a square blur is formed.
A linear pol in front will screw up the front slab that would normally 
result in parallel  streams of light with horiz and vertcially 
polarized steams---thus aliasing would occur on one axis.  Suspect that 
won't amount to a hill of beans unless the AA is strong and one has an 
image in just the right pattern
and perhaps that would not even be noticable.  Seems perfectly 
reasonable to try and use them

Mike









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