Definition: Measure of the increase in camera exposure needed when a filter is used. * Usually expressed as a factor -- i.e. used to multiply -- the exposure time e.g. a factor of 2X will double exposure time. * Filters absorp some light (usually the unwanted wavelengths) while transmitting the required wavelengths: the loss of light is compensated for by the factor, assuming the image underexposes the unwanted wavelengths while correctly exposing the required colour.
Previous Term: filter Next Term: F-line