Neutral density filters have four main uses:

• To allow the use of slow shutter speeds, especially with high-speed film, to record movement in subjects such as waterfalls, clouds, or cars.
• To reduce depth of field, allowing the use of larger apertures, which helps in separating subjects from their background.
• To reduce the effective ISO of high-speed film (above ISO 400) and enable its use outdoors in bright conditions.
• To allow film and video cameras (which have fixed shutter speeds) to record subjects such as snow, sand, or other bright scenes that could cause overexposure.

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)