The Hoya PRO1 Digital CPL 77mm Filter takes advantage of Hoya's patented optical glass technology to create a hardened glass that is thinner than the normal optical glass. This allows the overall thickness of the filter to be reduced to avoid vignetting when used with ultra-wide angle lenses. 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. Note! Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems and video cameras without problems); Pro 1 Digital filters are ultra thin but have a front thread.