Neutral density filters reduce or modify the intensity of light wavelengths. This allows the photographer to choose certain combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures Neutral Density Filters Have Four Main Uses: To enable slow shutter speeds, especially with high speed films, to record movement in subjects such as waterfalls, clouds or vehicles. 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 bright scenes.