An optical glass or resin filter placed in front of a camera's lens to reduce the brightness of the image.
A neutral density flat with a density of 0.3 would reduce the brightness by 1
stop. Also available in 0.15 (half stop), 0.6 (2 stops), 0.9(3 stops) 1.2(4 stops). Neutral density filters can also be graduated from clear to ND3, 6,9 etc so that part of an image can be controlled whilst leaving the rest unaffected. See
graduated filter.
