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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: "Ian M. Stewart" <ims@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 18 Apr 1998 20:50:40 +0100
Lars Bergquist <timberwolf@xxxxxxxxxx> explained to me:

> 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 <italic>subject,</italic> you must rotate the
> filter just as with a linear polarizer!

Thank you Lars, and the others who responded to my query.  My 
confusion arises from my earlier understanding that only circular 
polarisers can be used on those (non-Oly) autofocus cameras where the 
lens rotates as the autofocus operates.  (My son has one :-{  )

I assumed that this was because the orientation of the filter became 
unimportant when it was a circular pol - obviously I'm mistaken.

--

Ian  <IMS>
                  OM Tyro - please point my lens in the right direction.
mailto:ims@xxxxxxxxxxxxx

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