Cokin System Filters
Cokin filters are one of the most well-known manufacturers of camera lens filters on the market. Their range of filters primarily includes square filters that are held in place by a filter holder and attached to the camera lens by a threaded ring adapter.
This filter holder means you can use multiple filters at the same time in a stack, this type of filter means you can use Cokin filters across multiple lenses with the same adapter, rather than having to buy individual filters for each lens.
Cokin filters come in four classes, the A-series for compact cameras and camcorders, the P-series are the most popular and are for both film and digital SLR cameras, the Z-pro are larger and used for wide-angle SLR lenses and some medium format cameras, and the X-pro are even larger and used for medium format cameras and large format.