[Anvh]

PU = PentaxUser forum
http://www.pentaxuser.co.uk/forum/topic/why-are-polorising-filters-so-expens...p-0#comm183304

Try this test All about Polarizers - Linear and Circular
here is an easy test which can often indicate whether or not a camera needs a circular polarizer. Attach a linear polarizer to the camera and take a TTL meter reading off a blank wall illuminated by indoor (tungsten) lighting. Don't use any kind of shiny (reflective) surface. A carpet might also make a good target. Rotate the polarizer and see if the indicated exposure changes. If it does, you need a circular polarizer.

[newarts]

PU = PentaxUser forum
http://www.pentaxuser.co.uk/forum/topic/why-are-polorising-filters-so-expens...p-0#comm183304

Try this test All about Polarizers - Linear and Circular
here is an easy test which can often indicate whether or not a camera needs a circular polarizer. Attach a linear polarizer to the camera and take a TTL meter reading off a blank wall illuminated by indoor (tungsten) lighting. Don't use any kind of shiny (reflective) surface. A carpet might also make a good target. Rotate the polarizer and see if the indicated exposure changes. If it does, you need a circular polarizer.

That's equivalent to what I did to demonstrate the K-x's exposure doesn't change with angle as a linear polarizer is turned to for un-polarized light. The K100D has a 1/3 stop exposure variation as a linear polarizer is rotated.

Not all Pentax cameras are created equal regarding sensitivity to linearly polarized light. Linearly polarized light has no apparent effect on auto-focus or exposure for the K-x and 1/3 stop variation of exposure for the K100D.

[Anvh]

That's equivalent to what I did to demonstrate the K-x's exposure doesn't change with the angle a linear polarizer is turned to for unpolarized light.

Yes sorry, didn't read that to well.
Funny how it does effect the K10D and K20D and not the Kx, i wonder what is different.

[MetteHHH]

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.

kyc, thank you for the excellent link - that article is very readable, short, and answers the original question perfectly - even comes with an easy try-it-yourself test for linear polarizers. It does mention how AF could be affected in cameras with birefringent AF systems, but exposure would be a problem with any dSLR using a partially reflecting mirror. Here is the relevant quote:

So, do you need a circular polarizer? If you have a modern AF SLR the answer is almost certainly "yes". yes. Check your instruction manual. Some older manual focus SLRs also require circular polarizers. The camera manufacturers should be able to tell you if you don't have an instruction manual, or the manual doesn't say. What happens if you use a linear polarizer on a camera that really needs a circular polarizer? Basically you run the risk of exposure errors ( +/- 1 stop might be typical). This may not be a problem for print film, but can be a disaster for slide film. Not all exposures may be wrong, but some will be. There can also be small autofocus errors in some cases if any of the lenses in the AF system are birefringent (polarization sensitive), which can happen if plastic lenses are used and they are under some stress. If you care about your pictures and your camera needs one, spend the few extra dollars and get a circular polarizer.

[Eruditass]

Linear Polarizers worked fine on my K-x:

neutral density - Where can I find comparisons for linear polarizers (for a cheap variable ND filter)? - Photography - Stack Exchange

[philbaum]

Linear Polarizers worked fine on my K-x:

neutral density - Where can I find comparisons for linear polarizers (for a cheap variable ND filter)? - Photography - Stack Exchange

I haven't tried a linear yet, but from everything i've read on this thread, i'm much more likely to go for a linear polarizer for a large lens size than a circular one. In low light, one isn't likely to use a polarizer anyway. This is a much different story than the typical magazine article saying one can't use a linear on a dslr.