Circular -- for all types of cameras: required for beam splitting metering systems commonly found in auto-focus SLR's and in most current TTL SLR's.

Polarizers are most useful for increasing general outdoor color saturation and contrast. Polarizers can darken a blue sky, a key application, on color as well as on black-and-white film, but there are several factors to remember when doing this. To deepen a blue sky, it must be blue to start with, not white or hazy. Polarization is also angle-dependent. A blue sky will not be equally affected in all directions. The areas of deepest blue are determined by the following "rule of thumb." When setting up an exterior shot, make a right angle between thumb and forefinger. Point your forefinger at the sun. The area of deepest blue will be the band outlined by your thumb as it rotates around the pointing axis of your forefinger, directing the thumb from horizon to horizon. Generally, as you aim your camera either more into or away from the sun, the effect will gradually diminish. There is no effect directly at or away from the sun. Do not pan with a polarizer, without checking to see that the change in camera angle doesn't create undesirable noticeable changes in color or saturation. Also, with an extra-wide-angle view, the area of deepest blue may appear as a distinctly darker band in the sky. Both situations are best avoided. In all cases, the effect of the polarizer will be visible when viewing through it.