This is a P121F Graduated G2 Full Gray Neutral Density Resin Filter from Cokin. 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.
This filter measures 85 x 85mm and fits the Cokin P series holder.
The Cokin 121 series of filters comes in five variations of graduation.
Cokin 121 - ND8 3-stop Graduated Neutral Density
Cokin 121S -3-stop Graduated Neutral Density (Soft)
Cokin 121F - 3-stop Graduated Neutral Density (Full)
Cokin 121L - 1-stop Graduated Neutral Density (Light)
Cokin 121M - ND4 2-stop Graduated Neutral Density (Medium)