The Hoya 77mm Variable Density x 3-400 Filter means that you can adjust the brightness of a scene giving you more control over exposure settings during both still photography & video. For example, you are now able to use longer shutter speeds to capture smooth, blurred seascapes in full daylight. By rotating the inner filter ring, this filter reduces light transmission by Approx. 1/3 to 1/400th of its original level.