the special feature of a circular polarizer is the addition of a secondary layer on the camera side of the filter. this component is technically called a "quarter wave retarder" It is basically a very thin film whose thickness is one quarter of the wavelength of green light. green light is in the middle of the visible spectrum so it is expected the effect of this layer will catch most visible light. the surfaces of this layer are made to cause internal reflections. this means the light bounces around in the retarder layer a bit before it finds a weak point where it can get out. this bouncing randomizes the polarization of the light again so the light passing from the backside of the circular polarizer to the lens is now active in all directions.
why is this valuable? why is it even needed? the sensors used for autofocus and some flavors of light meters are sensitive to interference patterns. polarized light can develop subtle "bands" of light and dark that to us are just part of the image or maybe not even visible, but these silicon photosensors can sometimes see these patterns and will lock in on this interference pattern to determin the point of best focus and completely ignore the subject. it can also cause judgement errors in light measurement since SPD sensors do have a crystalline structure and polarized light might have a stronger effect in a particular orientation, again confusing the AF or metering system. so the circular polarizer was designed to eliminate this issue.
what else do these things do to my images? if you compare a scene through a circular polarizer vs a linear polarizer you may notice a mild color shift from one axis across the frame to another with a circular polarizer. also when comparing images taken with both kinds of polarizers fo similar quality, the contrast of the linear polarizer is a little higher and if you trust MTF testing it will measure a bit sharper. after all, the retarder layer is actually creating a bit of flare to do it's job. in real life, this won't matter unless you are doing some very demanding technical work, in which case you wonlt be relying on the in-camera automation so the linear polarizer will be fine.
basic rule... if your camera was designed to work with lenses that have a CPU in them, just spend the extra couple bucks for the circular polarizer.
2cents@large.
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