The Graduated Neutral Density Kit from Cokin consists of three filters: 121L (1 stop), 121M (2 stops) and 121S (3 stops). Often it is necessary or desirable to balance the light intensity in one part of a scene with another. This is especially true in situations where you don't have total light control, as in bright contrasty landscapes. Exposing for the foreground will produce a washed-out, over-exposed sky while exposing for the sky will leave the foreground dark and under-exposed. Cokin graduated ND filters are part clear, part neutral density. They allow the transition to be blended into the scene, often imperceptibly.
These filters have several uses and offers the possibility to achieve very interesting results. They enable the shooter to adjust exposure without affecting color balance. An ND filter appears gray and reduces the amount of light reaching either the sensor of a digital camera or the film plane of a traditional film camera.
A Neutral Density filter is used to create some unusual special effects such as capturing the "blur" of the rippling water of a waterfall or the swirling effect of city traffic. Since it allows the use of slower shutter speeds in bright light, it becomes possible to create these special effects that would normally not be possible to capture without the filter. And, since it can also help to control your depth of field, it permits you to shoot at wider apertures. Ultimately, its value is in helping to prevent bright, overly washed out images shot in bright light.
Cokin filters are used worldwide by pro and amateur photographers. They present an affordable alternative to more expensive filter systems.