Neutral density filter
A neutral density or ND filter is one designed to reduce the amount of light entering a lens. The term "neutral" implies that all colors of light are attenuated equally—thus, the filter appears neutral gray. Such filters permit wider apertures or longer exposure times to be used, even with brightly illuminated subjects—for example, those in sunlight.
The density of ND filters may be designated in one of several ways, depending on the manufacturer. For example, a density designated as 0.3 will reduce light transmission by one f-stop; 0.6 by 2 stops; 0.9 by 3 stops. etc. Alternately a "filter factor" may be quoted: The exposure time would be multiplied by this number to compensate for the filter's light loss.
Filters with a neutral-density gradient are also manufactured. One use for these is controlling the brightness of the sky in photos, while maintaining full exposure of subjects appearing below the horizon.
