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Neutral Density Filter
Neutral Density Filter
Also known as ND filter.
ND filter lowers the intensity of all wavelengths, or colours equally giving no changes in the hue of colour rendition.
It can be a colourless (clear) or grey filter, and is denoted by Wratten Number 96.
The purpose of an ND filter is to limit the amount of light entering the lens. Doing so allows the photographer to choose combinations of aperture, shutter speed and sensor sensitivity, which would otherwise result in an overexposed image. This is done to increase the depth of field or add motion blur.
For example, to take a picture of a waterfall at a slow shutter speed to create a motion blur effect, on a very bright day, even with the lowest aperture and film speed/ISO the image might be still too bright at a shutter speed of 1/10s. In this case, an ND-filter can be used to step down by one or more stops, allowing slower shutter speed and the desired motion blur effect.
For an ND filter with Absorbance d,
Fractional Transmittence = \(I/I_0\) = \(10^{-d}\)