Re: Which Polarizer?
- From: "AaronW" <bj286@xxxxxxx>
- Date: 31 Jul 2006 09:56:51 -0700
David Littlewood wrote:
In article <1154309188.102766.249040@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
AaronW <bj286@xxxxxxx> writes
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
Not random, but the polarisation is rotated a certain angle. Each color
is rotated a different angle. But a certain color is rotated a certain
angle. If your scene is mostly a single pure color, e.g., green, the
green color is just rotated, the result is the same as a linear
polarizer, just a different angle. You can try this yourself by
stacking 2 polarizers together, and see this effect. Circular polarizer
works best on greyish scenes.
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.
The beam splitter affects the AE/AF sensors, and the viewfinder. It is
partially polarizing.
Colour memory is notoriously fickle, and it is far more likely that it
is playing tricks on you.
Yes, but again you can stack 2 polarizers together and see the
exaggerated effect instantly, not on memory.
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.
The 1 stop is about the max. On average it is much less. And even
without polarizer, I change EC most of the time. With polarizer, my
changing EC is not that different.
http://digitcamera.tripod.com/#slr
.
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