The Hoya Variable Density filter uses two polarizing layers to control the amount of light that passes through the filter and into the camera lens. At its 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. After the filter is mounted on the lens, turning the filter ring between "Min" and "Max" can greatly control the amount of light entering the lens. With the Hoya Variable Density filter it is possible to shoot with fast lenses like a 50 mm f/1.4 lens wide open at f/1.4 in full sun for a very shallow depth of field. This is enough to create artistic blurring shots of motion on water, cars, people or almost anything that moves. The creative possibilities are endless.