Originally posted by Aristophanes You MUST use a circular polarizer for any digital sensor else the autofocus will not work. It's not the metering.
You need a circular polarizer (I don't think they even make linear polarizers anymore).
Check the lens database for filter diameters for each of your lenses. The 10-20 is a 77mm. The 28-75 is a 67mm. The DA 15 is a 49mm.
Brands I recommend are Hoya and B+W. You'll need a pricey thin filter for that 10-20mm or you'll vignette.
From what I've read, it's more an exposure issue (for some cameras, not all) than an autofocus issue.
See
All about Polarizers - Linear and Circular
The mirror in the DSLR splits the incoming light into two paths and linearly polarized light will be split with different intensities depending on the polarization. Thus, I guess it's possible that very little light gets to the AF sensor which stops it from working but as long as enough light gets to the AF sensor (which would be true most of the time) focusing functions should work.
However, different intensities of light (depending on orientation of the polarizer) hitting the metering sensor will definitely impact exposure.
I seem to recall reading somewhere that Pentax DSLRs may not be affected on exposure because it reads from a more direct light path than a split path, however I can't find that reference right now.