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Re: [OM] Identifying a filter and how to use it

Subject: Re: [OM] Identifying a filter and how to use it
From: Kennedy <rkm@xxxxxxxxxxxxxxxxxx>
Date: Sat, 18 Apr 1998 04:30:55 +0100
In article , Ian M. Stewart <ims@xxxxxxxxxxxxxxxxxxx> writes
>Jim Terazawa wrote about: Identifying a filter and how to use it
>
><snip>
>>.  The reason I can
>> tell they are circular pol is that they are inscribed "PL-C" on
>> the rotating collar (also they came with little 'instruction
>> manual' such as explaining how to use "V" triangle marker on
>> the collar - sorry I don't have the manual with me).
>
>I don't understand this (perhaps because I don't yet have a 
>polarizing filter).  I thought that the idea of a 'circular' 
>polarizer was that there is no need to rotate the thing, and 
>therefore it can be screwed on to the front of a zoom or whatever?  
>If so, then why is there a triangle mark and instructions about 
>rotating it?
>
>Sorry if I'm being dumb.
>
No probs :-)

No, a circular polariser is used in exactly the same way as a linear
polariser - it is rotated to get the desired effect, whether that is
elimination of haze, reflections or improved colour saturation on skies.

The difference - as far as the photographer is concerned - is only on
the effect it has on the cameras meter.  Some cameras (like the OM-3/4 &
2SP) have semisilvered mirrors in the light path to the meter and these
are partially polarising in themselves.  (To be pedantic - it is the
transmission through the primary mirror that is the main polarising
stage, although the secondary mirror also introduces polarisation since
it is not reflecting at 45deg.)  

Anyway, if an ordinary linear polariser was used then it would cause the
light reaching the mirror to increase and decrease as the filter rotated
since it would successively align and cross the polarisation of the
mirrors.  That makes the meter somewhat unreliable - around 2 stops or
so on my OM-4 for example.  The circular polariser outputs a special
form of polarised light that APPEARS to be unpolarised, so the meter is
unaffected.  However it is still only passing one polarisation axis of
the light which is coming from the scene you are photographing, just the
same as the linear device.

Hope this helps.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers         (replace 'nospam' with 'kennedym' when replying)

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