Archived posting to the Leica Users Group, 1999/04/14

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Subject: [Leica] How to use the M polarization filter properly?
From: Arne Helme <arne@huygens.org>
Date: Wed, 14 Apr 1999 11:15:52 +0200

Hi everybody,

I recently acquired an Leica M polarization filter for my M6 and I am
a little bit confused about how to use it correctly.  According to the
manual one turns the filter until the desired level of polarization is
obtained, swings it in front of the lens and then determines the
exposure normally by metering through the lens (and filter).  In this
way the exposure is dependent on the amount of polarization imposed by
the filter.  However, several places in literature I have read that
one should treat polarization filters as fixed factor filters (2.5x
unpolarized).  Thus, one determines the exposure *without* the filter
and then multiply by the filter factor (2.5x, always) to get the
correct exposure (unless some very dark areas that requires more
exposure are "uncovered" at maximum polarization).  Leica AG has
informed me that 3x is the correct filter factor at maximum
polarization.  Until now I have determined the exposure without the
filter and then increased it by 1 1/2 stop, and usually this method
works better than when measuring the exposure through the filter.  Any
opinions on this?

- -- Arne
| Arne Helme (arne@acm.org)     "We are all in the gutter, but some of us |
| http://www.huygens.org/~arne    are looking at the stars" - Oscar Wilde |
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