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.
Lee Grad ND filters are part clear, part neutral density (available in grades of .3 (1 stop), .6 (2 stops), .9 (3 stops)). They allow the transition to be blended into the scene, often imperceptibly. A neutral density .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.
Certain Lee graduated neutral density filters are available with soft or hard transitions from dark to clear. The soft edge is recommended for wide angle lenses and the hard edge is recommended for normal and longer lenses.
Lee graduated resin filters are 4 x 6" (100 x 150mm) x 2mm thick. The 6" length allows for adjustment of the filter within the holder.