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Re: [OM] B+W "Slim" Polarizer for 49mm

Subject: Re: [OM] B+W "Slim" Polarizer for 49mm
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Sun, 21 Jan 2001 01:21:58 +0000
At 04:15 1/21/01, John Hudson wondered:
ps: If by chance that my slim Hoya is in fact a linear poloarizer how
would I determine that?

John,
Linear polarizers only have a linear polarizer. Circular polarizers have a linear polarizer with a quarter-wave plate immediately behind it to circularly polarize the light the linear lets through.

There is a very easy test to determine if your polarizer has a quarter-wave plate behind the linear polarizer to make it a circular; one you can perform in the privacy of your own home. :-)

1.  Stand in front of mirror with polarizer in hand.
2. Close one eye and look through the polarizer at your reflection in the mirror using the other eye. (Closing the eye you're not using to look through the filter is important!)
3.  Now flip the filter over and look at yourself through it again.

Linear Polarizer:
You will see yourself in the mirror through it as if it's a mild ND in both directions. There won't be a difference looking through it one way or the other.

Circular Polarizer:
You will see yourself in the mirror through it in one direction as if it's a mild ND, just as with a linear polarizer. In the other direction (with the quarter wave plate facing the mirror), it will be blacked out entirely . . . or very nearly blacked out. The difference will be dramatic! The reason has to do with the quarter-wave plate on the back of a circular polarizer.


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