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. ND filters are also built-in to some light metering systems to vary the amount of light reaching the sensor, to compensate for film speed, or to couple the aperture or shutter to the meter.
Occasionally ND filters are employed to expose higher speed film in older cameras, which may not have high enough shutter speeds. ND filters can be stacked to further reduce light transmission but it should be noted that additional glass surfaces add to potential refraction and thus degradation in image quality.
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.
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. See the table below for other designations.
|Hoya ND Filter|
image by funkysuite (Image rights)
Table 1 Neutral Density values and codes used by filter manufacturers
|Reduction by f-stops||1/3||2/3||1||1 1/3||1 2/3||2||2 1/3||2 2/3||3||3 1/3||6||6 2/3||10||13|