Circular Vs. Linear Polarizers Filters

At a recent club meeting we had a discussion about camera filters, the question was asked what was the difference between a circular and linear polarizer filter. Here is a simple explanation (for more detailed scientific and quite confusing explanation one just do a search on google).

There are two types of polarizing filters available — linear or circular. Linear polarizers are more effective and less expensive than circular ones. But circular polarizers are needed with just about any camera that has a through-the-lens metering system, or autofocus.

The reason for this is that both of these systems use semi-silvered mirrors to siphon off some of the light coming though the lens. If that light is linearly polarized it renders either the metering or the autofocus ineffective. This means that you're going to have to buy circular polarizers unless you're shooting with a pre-1970's camera, or a view camera.
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