ThisHoya Multi-Coated Neutral Density (ND) Filteris an essential accessory for any photographer's camera bag. ND filters reduce the amount of visible light that enters the lens, allowing the user toutilize wider apertures or slower shutter speeds, particularly in bright conditions. ND filters work especially well for portraits by allowing the photographer toreduce the depth of field, producing pleasant background blur while keeping the subject in sharp focus. These filters are also often used toemphasize movement by permitting slow shutter speedsin bright light, giving life to photos that would otherwise look frozen in time. This particular filter (HMC) ismulti-coated to minimize reflectionon the filter surface, whichreduces flare and ghosting, and producessharp contrast and well-balanced color. An ND4 filter reduces the amount of light by2 stops. To enable slow shutter speeds to be used to record movement in subjects such as waterfalls, clouds, cars, etc. 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 (ie: above ISO400) and allow it to be used outdoors in bright situations. To allow video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which would normally cause overexposure.