Neutral density filters have four main uses:

• They allow the use of slow shutter speeds, especially with fast films, to capture movement in subjects such as waterfalls, clouds, or cars.
• They reduce the depth of field by allowing the use of larger apertures, helping to separate subjects from their background.
• They reduce the effective ISO of high-speed films (above ISO 400) and allow their use outdoors in bright conditions.
• They allow film and video cameras (which have fixed shutter speeds) to capture subjects such as snow, sand, or other bright scenes, which could cause over-exposure.

Neutral density factors:

• 2x = ND.3 (exposure adjustment = 1 stop, reduces ISO 1/2)
• 4x = ND.6 (exposure adjustment = 2 stops, reduces ISO 1/4)
• 8x = ND.9 (exposure adjustment = 3 stops, reduces ISO 1/8)