Neutral Density filters reduce the amount of light entering the camera lens without affecting the color. The ND.9 (also known as ND9 filter) reduces the light by 3 f-stop or ISO by 1/8. It can also be used to reduce the frame rate (fps) in a motion picture camera or the shutter speed in a still photo camera.
Neutral Density Filters
These filters reduce the amount of light passing through the camera lens without changing the color of the scene. They are especially useful in bright light conditions to help prevent overexposure. Neutral Density filters also allow proper exposure at a wider lens opening for reduced depth-of-field to highlight a key subject by making the foreground and/or background out of focus.