This Hoya Multi-Coated Neutral Density (Nd) Filter is an essential accessory for any photographers camera bag. ND filters reduce the amount of visible light that enters the lens allowing the user to utilize wider apertures or slower shutter speeds particularly in bright conditions. ND filters work especially well for portraits by allowing the photographer to reduce the depth of field producing pleasant background blur while keeping the subject in sharp focus. These filters are also often used to emphasize movement by permitting slow shutter speeds in bright light giving life to photos that would otherwise look frozen in time. This particular filter (Hmc) is multi-coated to minimize reflection on the filter surface which reduces flare and ghosting and produces sharp contrast and well-balanced color. An ND4 filter reduces the amount of light by 2 stops. When to use a Neutral Density Filter: 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.