Common photography myths (3)
Myth #2: Unlike linear polarizers, you don't have to turn circular polarizers
September 5th, 2009 - 08:30:42 PM:
The word “circular” may have caused this misconception. Of course you still have to turn a circular polarizer. Why would these filters have a rotating mount if you didn't have to turn them? The position of the polarizer selects the direction of polarization, and light which is polarized in this direction can pass the filter, and other light is reduced or even blocked. It's the entire point of a polarizer to be able to make that selection, so you have to and want to turn it.
What changes with a circular polarizer is the nature of the light leaving the filter. With a linear polarizer the light leaving the filter is polarized mainly in one direction. Light with different polarization was blocked by the filter. This can cause problems with cameras that use a beam splitter to lead some of the light towards the AF sensors and metering cells. These beam splitters also act like a polarizer. When the direction of the beam splitter is at a 90 degree angle relative to the direction of the polarizing filter, no or very little light will reach the sensors. The camera will have trouble focusing or may meter incorrectly, depending on the position of your polarizer in front of the lens. This is not what you want.
Circular polarizers solve this problem by adding a de-polarizing layer to the back of the
filter. This causes the light to be de-polarized, ie. it does no longer oscillates in a
single direction. This light can pass the beam splitter just like light that never
went through a polarizer, and AF and metering can work correctly.