"John A. Lind" wrote:
>
> 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.
John ........ I have to thank you most sincerely for this information.
My slim 49mm Hoya is linear, whereas my 67mm Hoya and 72mm Tamron are
both circular. The 67mm Hoya and 72mm Tamron have, respectively,
"PL-CIR" and "C-P.L" written round their edges.
It might be reasonable to conclude that if a PL does not have "PL-CIR"
or something similar written on one of the two rotating rings that it is
a linear PL.
Again, thanks for your information ......... it's truly appreciated.
John Hudson
Vancouver, BC
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