Neutral Density filters have several uses and offer the possibility to achieve otherwise unachievable results. ND filters appear grey and reduce the amount of light reaching the film. They have no effect on color balance. They have four main uses: 1) To enable slow shutter speeds to be used, especially with high speed films, to record movement in subjects such as waterfalls, clouds, or cars. 2) To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background. 3) To decrease the effective ISO of high speed film (above ISO 400) and allow it to be used outdoors in bright situations. 4) 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.
Neutral Density factors:
Cokin 152 (exposure adjustment = approx. 1 stop, reduces ISO 1/2)
Cokin 153 (exposure adjustment = approx. 2 stops, reduces ISO 1/4)
Cokin 154 ("P" series only; exposure adjustment = approx. 3 stops, reduces ISO 1/8)