Rodenstock 77mm f2 F STOP Filter ( 4x) 

These glass filters have a neutral grey tint and reduce all visible colours equally, e.g. by two f-stops. This allows motion blur (smear effect with panning camera) to be achieved with a longer exposure time than would otherwise be permitted by the given film speed. Alternatively, it allows less depth of field with a larger aperture.

x4 is equivalent to 2 stop