Rodenstock 77mm Circular Polarizer Multi-Coated Glass Filter -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.
Multicoated to minimize reflection at the filter surface, which reduces flare and ghosting.
Note:
Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems, without a problem)