Originally posted by lokerd 1) Does it matter to the k100d focusing & exposure system? I have heard that only some AF DSLRs need the circular.
I don't know about the K100D, but I've been using linear polarizers with my K10D without any problem. I don't think the K100D is any different.
Originally posted by lokerd 2) How can I tell a difference between the two?
Standing in front of a mirror. Cover one eye with the polarizer. Can you see still see the eye in the mirror? If not, it's a circular polarizer.
If you can see the eye behind the polarizer in the mirror, flip the polarizer over so that the face facing out toward the mirror now faces in toward your eye. Can you see the eye in the mirror? If not, it's a circular polarizer.
In other words, a circular polarizer transmits refected light only one way.
It's really simple. I hope my post does not make it too confusing.