Why circular polarizers?

Started May 11, 2014 | Questions thread
MediaArchivist
MediaArchivist Senior Member • Posts: 3,905
PDAF
1

Phase detect auto focus does not work (or at least not reliably) with a linear polarizer. A CPL uses a slightly different technique that yields the same polarization effect, but allows PDAF to work.

The Wikipedia article gets into the gory details:

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