In photography and cinematography, multiple exposures is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be identical to each other.
Ordinarily, cameras have a sensitivity to light that is a function of time. For example, a one-second exposure is an exposure in which the camera image is equally responsive to light over the exposure time of one second. The criterion for determining that something is a double exposure is that the sensitivity goes up and then back down. The simplest example of multiple exposures is a double exposure without flash, i.e. two partial exposures are made and then combined into one complete exposure. Some single exposures, such as "flash and blur" use a combination of electronic flash and ambient exposure. This effect can be approximated by a Dirac delta measure (flash) and a constant finite rectangular window, in combination. For example, a sensitivity window comprising a Dirac comb combined with a rectangular pulse is considered a multiple exposures, even though the sensitivity never goes to zero during the exposure.