linear vs circular polarizer

http://luminous-landscape.com/tutorials/polarizers.shtml

http://forums.dpreview.com/forums/read.asp?forum=1024&message=6913931:

A linear polarizer has a single plate for polarizing the incoming light. When this plate, which is attached to the filter, is rotated, the polarized light will change "phase/angle."

Some cameras, especially those SLR/DSLR bodies, have a beam-splitter to split the incoming light to the viewfinder, AF system and metering. Each of these components requires some minimal amount of light to work properly. For example, the AF system usually requires an aperture of F5.6 or larger. Thus, this splitting of light should follow a constant ratio, say 80% to viewfinder, 10% to light meter, and 10% to the AF system (just an example).

If the incoming light is NOT polarized, the phase/angle of the incoming light may be in all directions and evenly distributed to each of the three components. If the incoming light is polarized, depending on the way the incoming light is polarized, the phase/angle of the polarized light may be biased. In other word, since some portion of the incoming light is missing due to polarization, the light diverted to the three components may not be constant. For example, 70% to viewfinder, 5% to light meter, and 8% to the AF system. If the light is polarized significantly, the portion of the incoming polarized light that can reach the AF system may be too low to allow the AF system work properly. Or, if the light that reaches the light meter is lower than the expect level, the meter will "think" the scene illumination is insufficient and force the camera to use a larger aperture and/or a slower shutter speed. As a result, you might get over- or under- exposed images.

To overcome this problem, a second plate is added to a polarizer to "repolarize" the polarized light. The polarizers that have one plate are the "linear" ones, and the polarizers that have two plates are "circular" ones.

Therefore, if your camera does not have a beam splitter, you can use linear polarizers. Virtually all consumer level digicams do not beam splitter, and all SLR/DSLR bodies have beam splitters.

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