Neutral density filters reduce the amount of light going through the lens into your camera. By reducing the light you are able to use of slow shutter speeds, with high speed films or digital cameras, to record movement/image blur in subjects such as waterfalls or any fast moving subject. It also gives you full control over your depth of field. The Tiffen variable ND filter operates on the same principle as a circular polarizer - rotate until you reach your desired effect and shoot. It allows you to have continuous control over the amount of light coming through your lens in an approximate range of 2 (ND 0.6) to 8 (ND 2.4) stops.