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

[Lars Bergquist]

Wrote Ian Stewart:

>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?

Don't feel dumb--this is a bit complicated. When light has passed
trough a linear polarizer, it is (you guessed it) linearly polarized,
i.e. all the waves wave in the same direction. (Light waves are
transversal, at 90 degrees to the direction of propagation, not longi-
tudinal like sound waves.) Now, light is also partly polarized when
it passes through a semi-silvered mirror. Many TTL metering
cameras meter light which has gone through a semi-silvered spot
on the main mirror. If by chance the two directions of polarization
are close to 90 degrees from each other, the light will be partially
extinguished and the meter will give an incorrect value.

A circular filter polarizes light in two directions at a right angle
to each other, so that this effect disappears. But in order to get
a pola effect on the subject, you must rotate the filter just as with
a linear polarizer!


Vänliga hälsningar/Best regards
Lars Bergquist
Välkommen till/Welcome to ...
<http://www.bahnhof.se/~timberwolf/>

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