Neutral density filters have four main uses:

• They allow the use of slow shutter speeds, especially with high-speed film, to capture motion in subjects like waterfalls, clouds, or cars.
• They reduce depth of field by allowing the use of wider apertures, which helps to differentiate subjects from their background.
• They lower the effective ISO of high-speed film (above ISO 400) and allow its use outdoors in bright conditions.
• They enable video cameras (which have fixed shutter speeds) to capture subjects such as snow, sand, or other bright scenes that may cause overexposure.

Neutral density factors:

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