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 gray ND filters are part clear, part neutral density. They allow the transition to be blended into the scene, often imperceptibly. A ND.6-to-clear, with a two stop differential, will generally compensate the average bright sky-to-foreground situation and is also the most popular density of the series.
The Cokin 121 series of filters comes in five variations of graduation:
# Cokin 121 - 2 stop Graduated Neutral Density
# Cokin 121S - 3 stop Graduated Neutral Density (Soft)
# Cokin 121F - 3 stop Graduated Neutral Density (Full)
# Cokin 121L - 1.75 stop Graduated Neutral Density (Light)
# Cokin 121M - 1.75 stop Graduated Neutral Density (Medium)
This filter measures 130 X 170mm and only fits the Cokin X-PRO series holder.