Neutral density filters are often used to achieve motion blur effects with slow shutter speeds

In photography and optics, a neutral density filter or ND filter can be a colorless (clear) or grey filter. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition.

The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture, exposure time and/or motion blur of subject in different situations and atmospheric conditions.