In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colours, of light equally, giving no changes in hue of colour rendition.
It can be a colourless (clear) or grey filter, and is denoted by a number based upon the number of stops. The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens. Doing so allows the photographer to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures. This is done to achieve effects such as a shallower DEPTH OF FIELD (click to expand) or motion blur of a subject in a wider range of situations and atmospheric conditions.
A good video on the use of ND filters.
Examples ND use include:
Blurring water motion (e.g. waterfalls, rivers, oceans).
Reducing depth of field in very bright light (e.g. daylight).
When using a flash on a camera with a focal-plane shutter, exposure time is limited to the maximal speed (often 1/250th of a second, at best), at which the entire film or sensor is exposed to light at one instant. Without an ND filter, this can result in the need to use f/8 or higher.
Using a wider aperture to stay below the diffraction limit.