Often it is necessary to balance intensity in one part of a scene from 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 densisty. They allow the transition to be blended into the scene, often imperceptibly. A ND4-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.