Re: Which Polarizer?
- From: David Littlewood <david@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 31 Jul 2006 09:31:16 +0100
In article <1154309188.102766.249040@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, AaronW <bj286@xxxxxxx> writes
Hard to go along with what you are saying here.
The beam splitter is partially polarizing. That's why a linear
polarizer might interfere. But the AE metering is affected at most
about 1 stop. Since the meter is not perfect anyway, I might want that
1 stop exposure bracketing. And under certain circumstances (color,
angle, ...), circular polarizer will have exact the same AE problem as
linear polarizer.
For AF, the differences between circular and linear polarizer is very
little.
And because of the partially polarizing beam splitter, with a circular
polarizer, sometimes the color effect I see in the viewfinder is
different from that the sensor records when the mirror gets out of the
way. I noticed this problem with circular polarizer and switched to
linear polarizer.
First, a circular polariser can not have the same problems with a beam splitter as a linear one; the polarisation direction is effectively re-randomised after it has done its work. There may be some slight residual effect (the effect of the quarter wave plate is somewhat frequency sensitive) but nothing like that from a linear polariser.
And as for the sensor seeing something different from what your eye sees, this may be so, but it could not be because of the type of polariser and the effect of the mirror. The mirror is one of the types of surface ("specula", i.e. reflective metal) which has no surface polarisation effect. I suppose if you had an SLR with a glass mirror (Canon Pellix, EOS 1nRS etc) or an ancient rear-silvered glass mirror (Zenit) you may see some slight effect, but 99.9% of SLRs in the last 30 years use front silvered mirrors.
Colour memory is notoriously fickle, and it is far more likely that it is playing tricks on you.
And, as for the 1-stop effect of polarisation on the meter beam-splitter, I cannot even begin to understand why the fact that you *may* want to bracket should make this 1 stop factor unimportant. Metering is difficult enough to get right without cavalier dismissal of such an obvious source of unpredictable error.
David
--
David Littlewood
.
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