Light rays which are reflected become polarized. Polarizing filters are used to
select which light rays enter your camera lens. They can remove unwanted reflections
from non-metallic surfaces such as water or glass and also saturate colors providing
better contrast. The effect can be seen through the viewfinder and changed by
rotating the filter. The filter factor varies according to how the filter is rotated
and its orientation to the sun. Filter factor is between 2.3 and 2.8 (approx.
+1.3 stops).
Circular polarizers are specifically designed for use with auto-focus SLR cameras
(They will also work on manual systems without problems).