Only few years ago, photographers and videographers were asking for special effect filters.
Today, they ask for COKIN filters.
A French professional photographer, Jean Coquin, invented the concept of creative filter photography. To allow everybody to get a chance to produce great "creative images" Jean Coquin designed a system with the following prerequisites: simplicity, rapidity of use and versatility to leave plenty of room for individual inspiration and creativity.

How the square Cokin System works
The Cokin Creative Filter System consists of three main components:
A filter holder / Adapter ring and a Filter.
  1. Screw the adapter ring onto your lens.
  2. Slide the filter holder on the adapter ring until it snaps in place
    The filter holder is now solidly attached to the lens, yet it can rotate both left and right.
  3. Slide the filter into one of the filter holder slots.
    The one closest to the filter holder housing is the narrowest; it allows the use of round filters with notched edges (polarizer, star filters..). The two central slots are intended for square filters. The outer slot can be used either for an additional filter or for a coupling ring.
There are 4 different sizes of filters to choose from:
Choosing the right system Adaptor-rings are the first step in determining the system you should use. They are available in various diameters. First, refer to the lens with the biggest diameter you own. The diameter of a lens is either written on the front or side of the lens itself, in the cap, or in the lens user's manual. Once you know what is the diameter of your lens, refer to the illustration below to chose the system that will fit your needs.

Pure Harmonie -
Cokin introduce a range of
Pure Harmonie Round filters in UV, Circular Polariser and ND2.