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Neutral density filters (ND) reduce light transmission uniformly over the visible region of the spectrum, in definitive steps. Although used mainly in color photography for reducing light levels without color change, they can also be used for black and white photography. They are particularly useful for compensating for too much light, in circumstances where changing the shutter speed, aperture or film type is not possible or desirable. For example, exposure time can be increased to achieve special effects when photographing running water and waterfalls etc., without adjusting the aperture. In addition, depth of field can be reduced by increasing the aperture by the stop value of the filter used.
