Neutral density filters, on the other hand, are designed to greatly reduce light transmission, and have very high filter factors that permit very slow shutter speeds, even in bright daylight. 25 deep red filter for use with black and white film, are so dense that they require exposure compensation of four stops, while others like the UV filter are transparent and completely-neutral with respect to visible light, and therefore require no compensation at all. This additional exposure varies with the particular filter in use. When you do this, it is known as “exposure compensation,” because you are compensating for the filter’s effect on exposure.
This is achieved by opening the aperture wider or by increasing the time that the shutter is open. Since less light strikes the film/sensor, exposure must be increased to compensate for this loss. Most filters absorb some of the light that would reach the film or a digital camera's sensor if there was no filter in its path. Filter factors & corresponding stop values for daylight film.
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