Amazon.com Product Description
Cokin filters were invented by professional photographer Jean Coquin. By using square filters like Cokin's, which fit into a holder and are attached to a lens via an adapter ring, the photographer has a number of immediate advantages. There is no need to duplicate filters to fit lenses of different sizes, up to three filters can be used at once to create unique effects, and the photographer has control over the positioning of the effect on the photograph.
Product Description
Light rays which are reflected by any surface become polarised and polarising filters are used to select which light rays enter your camara lens. They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc. They also saturate colors and increase contrast. Polarizers come in two variations but have the same effect. Linear polarizers work with older camera meters; circular polarizers are designed for today's newer slpit-beam metering systems used in auto focus cameras; check your camera's manual. A160 Linear Polarizer A164 Circular Polarizer