The Urth 77mm ND8-128 (3-7 Stop) Variable ND Lens Filter Plus+ is a variable grey filter that reduces light by 3 to 7 stops, giving you more creative control in changing light. By blocking the light by 3 to 7 f-stops you can use a larger aperture or slower shutter speeds. This way you create a smaller depth of field or long exposure effects by using longer shutter speeds without overexposed images.