They have four main uses:

• To enable slow shutter speeds to be used, especially with high speed films, to record movement in subjects such as waterfalls, clouds, or cars.
• To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background.
• To decrease the effective ISO of high speed film (above ISO 400) and allow it to be used outdoors in bright situations.
• To allow cine and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which could cause overexposure.