The Heliopan 25.5mm Linear Polarizer Filter is 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: Linear polarizers are used with most video and manual focus photo cameras. They are not recommended for auto-focus SLR cameras. This filter eliminates ultraviolet rays to remove haze from outdoor shots as well as polarizing the light to remove reflections, and increase color saturation, without affecting the overall color balance. A linear polarizer differs from a circular polarizer in that it does not supports use of a camera's autofocus or auto exposure functions; it is best suited for use with manual cameras, such as view cameras, that are not dependent on internal mechanisms to determine focus or metering settings.