Originally posted by Squier So what are the optical differences in Linear and Circular PL's ? And why do so many folk use a Circular on a DSLR rather than a linear - i dont actually know anyone that uses a Linear
Without getting into the physics, which is beyond me anyway, a circular polarizer somehow depolarizes the light after going through the polarizing screen. This allows the light to go through the camera's optical path without cross polarizing on whatever sub mirrors are present (the AF mirror, for example).
As far as I can tell, people use circular polarizers because they have been told that is what they need by people who are in the market to sell more expensive equipment.
To be fair though, there may well be cameras out there who's exposures can be thrown off by a linear polarizer, it's just that Pentax isn't among them (with the one exception being the LX under certain usages).
We started getting circular polarizers in with the first wave of AF slr's back inthe 1980s, and we were told that this was all that would work, so that is what we sold. I think that it has elevated to urban legend status at this point.
Anyway, a linear polarizer may affect AF, causing it to hunt due to light loss if cross polarization occurs at the sub mirror, but with Pentax, it won't affect the exposure meter, which reads directly off the focusing screen.
Given that polarizers tend to be used in fairly static picture taking situations where AF can be turned off in favour of manual focus, I'm not seeing any advantage to circular polarizers