When I bought my polarizer, a friend told me that I should get a circular one, because the linear ones can mess with the autofocus. Is this true? What should each be used for?
Answer
A polarizer works in the way that it will let through only the light that is polarized in the same direction as the filter is currently turned. It is true that many AF systems have problems with this. To solve this, circular polarizers have a layer behind the linear polarizing filter that "re-polarizes" the polarized light in a different way so that the AF can function properly. The linear polarizer does not have this extra layer. The wikipedia article explains this quite well, I think.
As far as I know, there are no cases when you would want a linear polarizer instead of a circular one within the context of placing it on a camera lens. On any camera that a linear polarizer will do the job, the circular one will work just as well, but not the other way around.
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