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Re: [OM] linear polarizers?

Subject: Re: [OM] linear polarizers?
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Tue, 18 Jan 2000 03:59:34 +0000
At 05:31 1/18/00 , Joseph asked:
>Could someone give a reason why a circular polarizer is needed,
>or some evidence that it is other than just saying it says so in
>some FAQ somewhere?

This gets a little technical so there will be some generalities.

* How Linear Polarizers Work:
Linear polarizers are a single linear polarizer in a ring that allows it to
turn with respect to the male filter thread that screws onto the lens.
When it is turned so that its polarization is 90 degrees to the
polarization of any polarized light entering it, it blocks it.  At angles
between 0 and 90 degrees it will block some portion of it.  What passes
through it gets linearly polarized by the filter (even if it wasn't
beforehand).

* How a Sensor Behind the Mirror Gets Light to Sense:
For cameras with meter sensors behind the mirror, the mirror is
semi-silvered to allow some of the light to pass through it to the sensor
and reflect the rest up through the prism.  This is called beam-splitting,
and it too acts like a linear polarizing filter to the light that passes
through it to the sensor, and has a linear polarization angle.

* How the Combination Can Screw Up the Sensor:
If (emphasis on if) the angle of polarization of light making it through
your linear polarizing filter (having been polarized on the way through) is
anything but the same angle as the polarization of the mirror, some of it
that should pass through will be blocked.  Thus, the meter will think there
is less light than really is and the metering will show you need more
exposure than you really do.  How far your metering ends up being off will
depend on the difference between the angle of polarization of the (now
linearly polarized) light hitting the mirror and the angle of polarization
of the mirror.

* How Circular Polarizers Fix This:
Circular polarizers are two plates stacked.  The top one (the one facing
the subject) is a linear polarizing filter just like a linear polarizer
has.  It works to block polarized light just like on a linear polarizer.
The second plate under it (facing the camera lens) is what fixes
everything.  It is a "quarter-wave" plate.  The light that got linearly
polarized passing through the first linear plate, gets circularly polarized
when it passes through the quarter-wave plate.  How it does this goes way
too deep for here.  To a linear polarizer, circularly polarized light may
as well be un-polarized; it will not block any of it.  Thus, when it hits
the semi-silvered mirror none of the light gets blocked due to linear
polarization, the proper percentage gets through and your meter gives you
the reading it should.

* How to Test a Polarizer to See if it's Linear or Circular:
A linear polarizing filter will behave the same forward or reverse.  A
circular polarizer, if reversed will simply act like an ND filter.  Why?
The quarter wave plate circularly polarizes everything and the linear plate
(now behind it) will not block any of it.  Take your polarizer and look at
yourself through it in a mirror.  A linear will be equally dark forward or
reversed.  A circular will be significantly lighter if you're looking
through the back side (male thread side) than if you are looking through
the front (mine is virtually pitch black).

If you to get an idea of what happens with a linear polarizer and your
semi-silvered mirror, take two *linear* polarizers, stack them, and turn
one with respect to the other while looking through them.

Hope this helps explain the why.  If not, contact me off-list with any
further questions.

-- John

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