Tiffen's Ultra Polarizers polarize light "a little more" than regular polarizers.
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.
This filter is glass and measures 150 x 150mm (6 x 6") x 4mm thick.
NOTE: Linear polarizers are used with most video and manual focus photo cameras. They are not recommended for auto-focus SLR cameras. Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems and video cameras without problems).