[Leica] Question about A42 M Polarizer #13351

[henningw at archiphoto.com (Henning Wulff)]

>Hello Didier,
>
>To my documentation is a #13351 a A45 polfilter and a linear-type.
>The #13351 is a A42 push-on as well and also linear.
>
>May be can someone who's English is better than mine, explain what the
>difference is between circular and linear pol-filters.
>I can  explain this perfect in Dutch.....but I think this would not help
>Didier at all.
>
>Kind regards
>
>Fred Hess.
>
>
>
>
>----- Original Message -----
>From: "Didier Ludwig" <leica@screengang.com>
>To: "Leica Users Group" <lug@leica-users.org>
>Sent: Friday, October 26, 2007 1:05 PM
>Subject: Re: [Leica] Question about A42 M Polarizer #13351
>
>
>>  Fred an Hoppy, thanks a lot.
>>  Hmmm, so far I believed a pol filter like this #13351 is a circular 
>> filter
>not a linear so don't ask me about the difference ;-) but I will be pleased
>to learn it.
>>  Didier
>>
>>
>>  >Its a liniar pol filter, the #13351 and you are correct: The filter is 
>> in
>>  >the same pol-position in the 0 and 180-position.
>>  >By the way: do you know the difference between circulair and liniar
>>  >pol-filters?!
>>

Right, it's a linear polarizer. The M's don't 
need circular ones. The circular ones are for 
cameras that use semi-silvered mirrors as part of 
their operation, either for metering such as the 
Leica reflexes or for AF. In those cases the 
polarized light coming through the back of the 
filter can interfere (cross-polarize) with the 
polarization that occurs in the semi-silvered 
mirror and a) cause overexposure or b) not AF 
correctly or at all.

The circular polarizer has a so-called 'quarter 
wave plate' at the back, which depolarizes the 
light again. That's OK, because it's the 
relationship of the reflection of the light off 
non-specular parts of your subject and the 
polarization direction of the entrance of your 
filter that causes the effect, and if it gets 
de-polarized after that it doesn't matter to the 
effect on the film or sensor.

Circular polarizers are somewhat more expensive 
to make, but don't create a better polarization 
effect.

As mentioned before, 0? is the same as 180?, for either type.


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    *            Henning J. Wulff
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