Originally posted by goubejp You need a circular polarizer, cause linear polariser are not compatible with the phase detection AF system; old polarizer are most of the time linear polarizer; you can solve this using live view focus. I have also read on polariser tests that some filters marked as "circular polariser" were linear.
Regards
Linear pols work just as fine. All they might do is to reduce the available amount of light for the AF system - which would only be severe under dim lighting. And that is a condition where it is quite unlikely to use a polarizer anyway.
Just do a search on this topic. This has been discussed in depth several times this year and both, the theoretical explanation and the practical user experiences show, that Linear Pols are just as useable with Pentax DSLRs.
Ben