polarizer filter

from http://www.kenrockwell.com/tech/filters.htm

There are at least two kinds of polarizers. The conventional kind is now called a linear polarizer. It works great with all rangefinder cameras, view cameras and most manual focus cameras. You don't need to know, but they are called "linear" because the light waves coming out the back of them vibrate in one plane in a linear fashion.

Most autofocus cameras use polarization tricks as part of the autofocus system. Therefore if you use a regular linear polarizer you may mess up your focusing and metering. Because of this a new kind of polarizing filter was developed, called a circular polarizer, that eliminates these problems. No, these names have nothing to do with the shape of the filter! Circular polarizers use a trick (a quarter wave retardation plate if you must know) that makes the light waves coming out of the back of the filter spin around in a way called "circular polarization." This means that the light coming out the back of a circular polarizer does not appear polarized to the camera, and therefore it won't confuse anything. Yes, the effect of the two filters is the same on your pictures. The circular polarizer costs more and may be used anyplace you'd use a linear polarizer. You can test your filter by looking through another POL filter (or POL sunglasses) through the back of the filter in question. Rotate the filter in question. If it gets darker and light er as rotated then it's not circularly polarized. It should always look the same. Turn the filter in question around and look through the front of it as you rotate. It will now go dark and light, confirming that you're running the test correctly.