B+W 77mm Colour Grad Grey 50% filter (501 63811)

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. For example, when the sky is too bright in relation to the landscape, this is an ideal amount for good detail rendition in the clouds and for preventing the sky from being “washed out” by over-exposure.

The info and sample effect picture are taken from schneideroptics.com