Neutral density (ND) filters reduce the amount of light entering the camera, enabling a longer exposure time than otherwise possible. This can emphasize motion, or make an otherwise tumultuous scene appear surreal and quiescent. Alternatively, an ND filter also enables larger apertures, which can produce a shallower depth of field, or achieve a sharper photo. Either way, this is a useful and often under-appreciated filter that deserves a deeper look. ND filters are also one of the easiest filters to use, and their effect cannot be replicated digitally — at least not with a single shot.