Light rays which are reflected become polarised. Polarising 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 colours 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).
Linear polarisers are used with most video and manual focus photo cameras. They are not recommended for auto-focus SLR cameras.
PLEASE NOTE: All auto-focus SLR and certain manual focus SLR cameras require circular polarisers. Consult your instruction manual.