The Hoya 72mm Variable ND Neutral Density Filter uses two polarizing layers to control the amount of light that passes through the filter and into the camera lens. At it’s minimum effect the filter passes 1/3 of the light in a scene. That is equal to 1.5 stops on the aperture or shutter speed. At it maximum effect the filter passes just 1/400 of the light in a scene. That is equal to 9 stops on the aperture or shutter speed.