Multiple exposure

In photography, a multiple exposure is when two or more individual exposures are made to create a single photograph. 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 a multiple exposure 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 exposure, even though the sensitivity never goes to zero during the exposure.
In film and photography, double exposure twice, to two different images. The resulting photographic image shows the second image superimposed over the first. The technique can be used to create ghostly images or to add people and objects to a scene that were not originally there. It is frequently used in photographic is a technique in which a piece of film is exposedhoaxes. It also is sometimes used as an artistic visual effect, especially when filming singers or musicians.
Digital photographs can be superimposed by using a software photo editor such as Adobe Photoshop or GIMP, by altering the opacity of the two images and line them up over each other, or set the layers to multiply mode, which 'adds' the colors together rather than making the colors of either image pale and translucent.
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