On polarization from a 5 year old post by John Lind which I carefully
squirreled away. BTW, where is John? I don't recall seeing any posts
from him for quite awhile. Probably doesn't have any OT stuff to post.
Chuck Norcutt
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Date: Tue, 18 Jan 2000 03:59:34 +0000
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Subject: Re: [OM] linear polarizers?
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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Jim Brokaw wrote:
>
> I seem to remember that one easy way to determine if the polarizer is linear
> or circular is to view your own reflection in a mirror through the polarizer
> as if it were a sunglass lens... Than flip it around and view your
> reflection again. If the polarizer is lighter one way than the other then
> its linear... or is that circular? Anyway, it has something to do with how
> the light comes through the polarizer twice off the mirror. Somebody here
> with a real, good memory will remember this correctly.
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