The hoya ndx400 hmc filter is designed to photograph with a wider aperture or slower shutter speed than normally required it does this by slowing exposure time or increasing aperture enabling control depth of field and conveys movement more easily it has a solid neutral density which allows a 9 stop surge in exposure length and darkens the entire image with its optical 27 density the 9 stop light reduction is equal to a nd optical density of approx 27 neutral density filters are