This is a high quality glass filter made by HOYA. The Circular Polarizer is the choice of most Auto Focus camera users. There is no need to adjust the amount of polarity the filter has after focusing, so it is faster than using the Linear Polarizer. Light rays that are reflected become polarized. Polarizing filters are used to select which light rays enter your camera lens. They 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. Circular Polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems and video cameras without problems). This filter is threaded front and back. You can screw another filter to it or another accessory like a lens hood.