Glossary Terms
A Neutral Density filter (or "ND filter") is a grey glass or plastic filter designed merely to reduce the level of light - with no effect on colour. ND filters on a lens allow slower shutter speeds or wider apertures to be used, to give motion blur or reduce depth-of-field. ND filters are also built-in to some light metering systems to vary the amount of light reaching the sensor, to compensate for film speed, or to couple the aperture or shutter to the meter. Occasionally ND filters are employed to expose higher speed film in older cameras, which may not have high enough shutter speeds.