I'm not positive I'll explain it right, but I think I understand it in a general sense, so here goes:
"Naturally", light is not polarized. It travels in waves that can travel oscilating along any axis. Polarized light occurs when light reflects of a smooth surface such as water. The light from most axis are absorbed, but the light along a single axis, parallel to the surface, is reflected. A linear polarizer simply filters out light along a single axis. You adjust the polarizer so the axis on the filter matches the axis of the polarized light, and the glare gets filtered out. I believe at that point, the light is polarized to some degree on the perpendicular axis. As Gordon explained, certain auto focusing systems (I believe those that rely on a prism) have a problem focusing with a linear polarizer - I don't recall the reason. What a circular polarizer does is addes a second layer behind the first, which has the effect of "spinning" the polarized light from the first layer, creating the effect of non-polarized light.
I can't argue the validity of this, but from what I've read, this is primarily an issue with TTL auto-focus systems, particularly SLRs. I think it's been determined that non-dSLR digicams don't need a circular polarizer. Circular polarizers are more complex (in a relative sense), and depending on the quality and manufacturer can cost anywhere from slightly to significantly more than a linear polarizer. On the other hand, as far as I know, any camera can use a circular polarizer without a problem, so you can't go wrong with that choice, and you potentially have a filter that can be used if you upgrade to a dSLR in the future.
Here's a link that probably has more than you'd ever want to know about polarizers: DPFWIW Using a polarizer.... There are a couple other links on another thread posted today that are a bit more concise. |
