This filter from Fotodiox is a double threaded circular polarizer (CPL) filter made of optical glass and featuring a rotating ring to allow for light ray selection. Remove unwanted reflections from non-metallic surfaces such as water or glass. CPL Filters are used to select which light rays enter your camera lens. CPL filters can also be used to saturate colors and to provide better contrast. The effect can be seen through the cameras viewfinder and changed by rotating the filter ring. The filter factor varies according to how the filter is rotated and its orientation to the sun or light source(approx. +1.3 stops). Typical applications include darkening skies, water reflection elimination and window/glass/eyeglass reflection elimination. CPL filters are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems and video cameras without problems). Linear polarizer filter may not work properly with auto-focus cameras. Each filter comes with a plastic storage box.