Neutral Density filters have several uses and offer the possibility of achieving otherwise impossible results. ND filters appear gray and reduce the amount of light reaching the film. ND filters also do not have an effect on color balance.
Four main uses:
•Enables slow shutter speeds to be used, especially with high-speed films, allowing film to record movement in subjects such as waterfalls, clouds, or cars.
•Decrease depth of field by allowing wider apertures to be used, in turn helping to separate subjects from their background.
•Decrease the effective ISO of high-speed film (above ISO 400) and allow it to be used outdoors in bright situations.
•To allow picture and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes that can cause overexposure.