B&W 52 mm Graduated #501 Neutral Density (ND) .3 (1-stop) Glass Filter - The neutral gray half of this filter transmits 50% of the incoming light so that it darkens the respective portion of the subject by one f-stop without altering its colors.
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 contrasting 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. This filter enables cloud detail to be kept correctly exposed in the picture.
Determining which graduated neutral density filter yields ideal results for any given lighting situation takes knowledge, experience and a collection of such filters. Choose the filter strength which adjusts the lighting to stay within the exposure latitude (greatest difference between bright/dark values) which still shows details in both of the digital or film medium in use.
Speaking generally, the 2-stop value (ND 0.6 - the filter's clear portion allows 4x more light to pass vs. darkest portion) effectively compensates average bright sky-to-foreground situations.