CN writes:
>>There are two types of polarizing filters readily available, linear
and
>>"circular", which have exactly the same effect photographically. But
the
>>metering and auto-focus sensors in certain cameras, including
virtually
>>all auto-focus SLRs, will not work properly with linear polarizers
>>because the beam splitters used to split off the light for focusing
and
>>metering are polarization-dependent. Linearly-polarized light may
also
>>defeat the action of the Anti-aliasing filter (Low-pass filter) on
the
>>imaging sensor.
I am not convinced linear and circular are "exactly" the same
photographically all things being equal.
The extinction coeeficients of the polarizing material vary by wave
length, thus the blue shift. The extent depends on the
exact brand/material---see the pol tests links I provided.
To quote Roger Clarke:
"The visible spectrum, violet to blue, to green, yellow, orange, and
red, covers about a factor of 2 in wavelength. That means that the
circular polarizer will not make perfectly circularly polarized light
for all colors. At wavelengths away from that designed for the
quarter-wave plate, the light will be elliptically polarized. What this
means is that as you rotate the circular polarizer when viewing a
polarized source through your camera, there will be a slight color
change. For example, on polarizers I have used, a scene with a lot of
polarized light becomes slightly "warmer" when a circular polarizer is
used (this means a color shift to the red). Such small color shifts can
be easily corrected, if desired, by a change in white balance in
digital camera images. For mountain scenes, or scenes with a lot of
blue sky illuminating the scene, in addition to the sun, the warming
color can help make a more pleasant image. "
Thus the circ pol may have less blue shift even if it had the same
effect quashing reflections--thus the origin of the impression that
linear pols
have more effect. (that is my theory anyway) The quarter wave retarder
is not color neutral.
Good point though as no semi silvered mirrors that caused trouble with
linear pols (even in OM4t) and PDAF in DSLR are present in mirrorless,
so why wouldn't linears be fine. I don't understand the effect of
linear pol on the AA filter though and perhaps the explanation will
result in a headache but am curious.
Not a physicist, Mike
--
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