Polarizers are universally popular filters and their main use is to eliminate unwanted reflections from surfaces such as water and glass. They also make images appear clearer, with better contrast and increased color saturation, without altering the overall color balance. Polarizing filters will also darken blue sky and enhance detail in clouds.
Polarizers are available as linear (PL) and circular (CPL) and need approximately 2 stops of exposure compensation. Linear polarizers can cause errors with TTL light meters and autofocus systems, whereas circular polarizers do not. CPL filters are designed specifically for use with autofocus and TTL metering of SLRs.
A CPL filter is made of two glass elements (a linear polarizer and quarter-wave plate), the front of which rotates 360°. Rotating it clockwise or counterclockwise changes the amount of polarization by allowing some rays through and blocking others. The effect of a CPL filter can be seen in the viewfinder of an SLR but not in the viewfinder of a rangefinder camera, for which PLs are better suited.