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In photography, a multiple exposure is an exposure in which the sensitivity to light is reduced and then increased at least once during the total exposure time.
Overview
Ordinarily cameras have a sensitivity to light that is a single rectangular 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 a multiple exposure is a double exposure without flash, i.e. the camera image is responsive to light twice during the 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 exposure, even though the sensitivity never goes to zero during the exposure.
Film based multiple exposures
Photographic film is sensitive to light in an integrational way, i.e. functions like a true integral over time, and therefore "sifts out" the effect of a delta measure (flash).
C.E.M.E.N.T.
With electronic imaging, long duration sensitivity is not practical, so multiple exposures are usually made using CEMENT (Computer Enhanced Multiple Exposure Numerical Technique). This simulates the effect of multiple exposures numerically, by using a computer, or computation.
CEMENT is described, in detail, in Chapters 4, 5, and 6 of Intelligent Image Processing (John Wiley and Sons).
The use of CEMENT as a medium of artistic expression dates back to the 1970s and early 1980s with the invention of the wearable computer as a tool for visual artists. Using CEMENT for the production of visual art, especially when walking around with a flash lamp, is called lightvectoring. A more colloquial term for lightvectoring is dusting.
Long exposures
With traditional film cameras, a long exposure is a single exposure, whereas with electronic cameras a long exposure is best attained by integrating, averaging together many exposures. This averaging also permits there to be a time-windowing function, such as a Gaussian, that weights time periods near the center of the exposure time more strongly. Another possibility for synthesizing long exposure from multiple-exposure is to use an exponential decay in which the current frame has the strongest weight, and previous frames are faded out with a sliding exponential window. Such a setup is used on webcams such as the "Time AGO" webcam (see links below).
External links
"Multiple exposure"