Schneider Optics 4.5" Round Drop-In Neutral Density Filter 0.3 is a Neutral Density Filter used for Cinematography. A common use for this filter is to allow the use of larger apertures, reducing depth of field, to allow selective focus and rack focus.
Neutral Density Filters 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 failm subjects such as snow, sand or other bright scenes which could cause overexposure