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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