Re: Which Polarizer?

In article <1154365011.696392.242080@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, AaronW <bj286@xxxxxxx> writes
David Littlewood wrote:

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.

If you think a quarter-wave plate simply rotates the plane of polarisation, then you have clearly not understood birefringence; it is rather more complex than you suggest, and certainly does *not* involve the rotation of polarised light (which is an entirely different property).

However, for all practical purposes you can regard the light, after passage through the quarter-wave plate, as *effectively* re-randomised (which is why I used exactly those words earlier). More precisely, the polarisation vector is rotating trillions of times a second, far faster than can be seen by any meter, so it sees the average, i.e. "random", vector direction. As I said (in the bit you carefully snipped) there is a residual effect as the birefringence is somewhat colour sensitive, but this is a much smaller effect for the issue here.

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,

Yes; I think we all agree on this.

and the viewfinder. It is
partially polarizing.

By what mechanism do you believe this to occur? Not something I have seen suggested before. The beam splitter is not in the optical path to the viewfinder - you did realise that?

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.

No; I just tried it, and could only see a slight decrease in density (because of the absorption of the polarisers). No colour change was apparent. If you use polarisers with a slight tint, this would obviously double if you use 2. Using 2 polarisers does not even increase the polarising effect (unless they are very inefficient polarisers). Otherwise, I think it's in your imagination. There is also, BTW, no obvious theoretical reason I can see why your claim should be valid, but if you know one I would be interested to hear it.

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.

I don't disagree with you assessment of the amount - I just did a test and found the difference was typically 1/3-1/2 stop. However, there are enough variables without ignoring things easily controlled.

David
--
David Littlewood
.