Archived posting to the Leica Users Group, 1997/03/21

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Subject: Re: Circular/Linear Polarizers
From: Kari Eloranta <eloranta@lammio.hut.fi>
Date: Fri, 21 Mar 97 14:38:42 +0200

"Peter Jon White" <pjwhite@tiac.net>:

>Kari said,
>> Linear works very simply as well. It is a unfortunate  
>>misconception  that linear polarizer doesn't work with beam  
>>splitter meters.

<snip>

>While this may work in many situations, it can easily lead to
>incorrect exposures.

>For instance, while the polarizer has a neutral density of about a 
>stop, it affects polarized light far more than one stop. If the sky 
>is what you are darkening with the polarizer, you need to know it's 
>value through the polarizer in the position it will be in when the 
>image is made.

Read my post and think the phenomenon at hand through. A meter with  
a beam splitter will always polarize the incoming light before it  
hits the silicon diode or CdS cell or whatever. Point the lens to  
the zenith at a clear day and rotate the camera to see this. So if  
your subject is throwing heavily polarized light at you, you must  
always be careful with a meter of this type. A polarizer at the  
"neutral" position (max as described previously), where it does not  
add any polarized wave directions, adds no further complication to  
the exposure determination.

>In order to really know the effect the polarizer is having with this 
>type of camera, you must have a circular polarizer.

Absolutely no. If you care about the physics, try it in practice.  
I've been shooting with a camera with beam splitter meter (Ftb,  
F-1s) for some twenty years. Polarizer used as I described works  
perfectly in *every situation* indoor or outdoor. If you can  
determine the exposure with a meter like that in the first place  
i.e. without the polarizer, with the method described you can do it  
with the polarizer as well. Some situations can be tricky *for the  
meter* and then it is better to bracket.

The bottom line is that with a meter like that you need to be aware  
of the situations, where polarized light might fool you. The filter  
adds no complication.

In your specific example the desired darkening of the sky usually  
amounts to more than one or two stops. But this is beside the point.  
You see the effect on the sky in the screen *after you've set the  
exposure*. The exposure reading you take from someone's face, some  
detail in the landscape etc., hopefully some non-polarized source.  
If you for some reason take it from the sky or some polarized source  
you need to worry just the same amount with or without the filter.

>Peter Jon White

Kari Eloranta