Re: Which Polarizer?

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

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.


Not rotated, but rotating thus its angle is continuously changing; in other words "circularly polarized".

It probably isn't clear what you are saying. (not that this will
help much, but here goes) When light goes through a linear
polarizer, the light (e.g. the electric vector of the electromagnetic
wave) vibrates in a single plane. Light vibrating 90 degrees
from that are blocked from passing through the polarizer.
Think of carrying your tripod with the legs extended, and you
walk through some vertical bars. If you have the tripod vertical,
you can go through, if horizontal, you get blocked.
Now think if the tripod legs as the direction of vibration
of light waves.

In a circular polarizer, you have a linear polarizer in front
and a "quarter wave plate" in back. The light that passes
through the linear polarizer then passes through the plate,
which has indices of refraction different along two
orthogonal axes. The thickness of the plate is such that along
one axis, light of a specific wavelength is slowed 1/4 the
wavelength. Then when the polarized light passes through the
plate, oriented so the axes are 45 degrees to the linear
polarizer, the electromagnetic wave vectors end up rotating.

Back to the tripod analogy: after passing through the vertical bars,
with the tripod vertical, you then pass through the quarter wave plate
and come out the other side twirling the tripod as you
continue to walk. (you are twirling the tripod orthogonal
to the direction you are walking). A subsequent encounter with
a polarizer (linear or circular) acts as if the incident light
is unpolarized.

Regarding other statements made, Mirrors, even 100% aluminum,
still polarize light, as do beam splitters. I have the data
in my office,and if I remember I'll look it up.
It is significant enough that I have had to compensate for
it in spectrometers I have designed. That is the reason why
a circular polarizer is needed: the multiple reflections,
and the AF and meter system can be influenced (i.e. get
the wrong data) with polarized light. The circular polarizer
reduces that problem.

Regarding the two circular polarizer experiment: if you take
two circular polarizers and stack them then rotate one,
you should see NO change in intensity, and NO color change.
If you do, they are not quality circular polarizers.

See:
http://www.clarkvision.com/photoinfo/evaluating_polarizing_filters

Roger
Photos at: http://www.clarkvision.com
.

Relevant Pages

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  • Re: Is Circular Polarizer Really Necessary for DSLR?
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  • Re: Which Polarizer?
    ... >The beam splitter is partially polarizing. ... That's why a linear ... >polarizer might interfere. ... I noticed this problem with circular polarizer and switched to ...
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  • Re: Linear polarizers not supposed to work on DSLRs?
    ... A circular polarizer will do similar things, ... affecting camera operations. ... If you meter and focus automatically, screw it, use a linear. ...
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