From: "John A. Lind" <jlind@xxxxxxxxxxx>
Reply-To: olympus@xxxxxxxxxxxxxxx
To: olympus@xxxxxxxxxxxxxxx
Subject: Re: [OM] Re: Metal surfaces and polarizers
Date: Sun, 28 May 2000 00:51:22 +0000
Looking through the viewfinder, the circular will behave just as a linear
does (except perhaps for metering on an OM-2S[p], OM-3 or OM-4). A crash
course on polarizers:
(1) Light normally encountered is polarized nearly equally in all
directions.
(2) Glare from light reflected off glass, water, etc. tends to be linearly
polarized in a specific direction. The same applies to blue sky. The
highest percentage of specific polarization direction occurs at about 90
degrees to the light source (this is _not_ the reflection, but the source
itself).
(3) A linear polarizer admits only light that is polarized in a specific
direction (actually two directions 180 degrees apart). That is why you
have to be able to turn it. By making the light admitted by the polarizer
90 degrees out from the polarization of the glare, it blocks it, and that
is why the glare decreases and increases as you turn the polarizer. Note
the importance of being at the proper angle to the light source (not the
reflection of it, but the original source).
(4) If you look at a linear and circular side by side you will notice the
linear has a single piece of glass which is mounted in the rotating ring.
The circular has two. One is mounted in the rotating ring and the other
fixed in the part that screws into the lens ring. On a circular, the part
that rotates is a linear polarizer, just as with your linear one. The
fixed, second piece of glass behind it is a special filter that circularly
polarizes the linearly polarized light that was admitted by the linear
polarizer, effectively dispersing it into the same as linearly polarized in
many directions. Your eye doesn't care about the polarization. That is
why when the first one blocks the glare, it doesn't matter that the second
one effectively disperses the polarization into many directions again.
(5) Why use a circular polarizer with the extra filter? Many newer SLR's
meter and/or auto-focus with sensors behind the mirror instead of in the
prism (as the older ones did). Most of the light is reflected by the
mirror, but some is admitted through it to the sensor(s) behind it. These
mirrors are more appropriately called a "beam splitter." How does it do
this? It is essentially a linear polarizer reflecting all but a specific
amount of light polarized in a specific direction. It assumes that the
light striking the mirror is polarized in all directions equally. If you
use a linear polarizer, it is not. If for some reason your linear is
turned 90 degrees to the polarization of the "beam splitting" mirror, then
your exposure will be off (and auto-focus will have difficulty). Likewise,
exposure could off if it is aligned to it. By having the additional filter
to circularly polarize the light surviving the linear in front of it, the
nearly equal dispersion of polarization direction allows the mirror to work
as originally intended.
(6) If your camera (such as an [O]M-1[n] or OM-2[n]) does not have a "beam
splitting" mirror, then use a linear. The extra piece of glass does
nothing for correct metering and only adds another piece of glass for
absorption, distortion and diffusion, and two air-glass surfaces for flare.
-- John
>My two 49mm polarizers are a Tiffen circular polarizer and a Vivitar that
>is presumably a linear polarizer. Both have indicator dots and must be
>turned 45 degrees to achieve polarization. Should'nt the circular one be
>"active" at all settings and not require adjustment??
>
>>From the film results I've seen polarizers are perhaps the best thing
going
>for both more vivid color and for better contrast in B&W, but I see
little
>mention of them as being as useful as they are. Is there any merit in
>paying for top brand ones, or are they pretty much all alike?
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