Multiple exposure
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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.
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[edit] Overview
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. 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.
[edit] Double exposure
In film and photography, double exposure is a technique in which a piece of film is exposed 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 hoaxes. It also is sometimes used as an artistic visual effect, especially when filming singers or musicians.
It is considered easiest to have a manual winding camera for double exposures. On automatic winding cameras, as soon as a picture is taken the film is typically wound to the next frame. Some more advanced automatic winding cameras have the option for multiple exposures but it must be set before making each exposure. Manual winding cameras with a multiple exposure feature can be set to double-expose after making the first exposure.
The problem with double exposures, is that if too much film is exposed over the 2 shots, the picture will turn out overexposed. Medium to low light is ideal for double exposures. Also, it is important to use a tripod for double exposures. If the setting of the two shots do not line up almost exactly, you won't get the desired effect and the background will be blurry looking, as well as the target.
Double exposures are not the limit, as multiple exposures are possible, but the photographer should try not to overexpose the film.
Another way to superimpose pictures is digitally, using a software photo editor like Adobe Photoshop or GIMP. Simply alter the opacity of the two images and line them up over each other.
[edit] Examples of Double Exposure in Movies and Television
- On the Ed Sullivan Show, this effect was sometimes used with singers and rock groups. An image of the stage set was superimposed on the performer(s).
- In the movie Mary Poppins, when Julie Andrews is singing "Feed the Birds" to the children, there's an instrumental section of the song. During this section, an image of St. Paul's Cathedral is superimposed on Julie Andrews' face.
- On the popular 1960's situation comedy The Dick Van Dyke Show, there's a special Christmas episode called "The Alan Brady Show Presents" . On that episode, the double exposure effect is used on Larry Mathews (who played Dick Van Dyke and Mary Tyler Moore's cute young son, Ritchie). He's singing the Christmas song "Little Drummer Boy", and at one point, an image of the stage set (a stained glass window) is superimposed on his face.
- On a current commercial for Fruit of the Loom underwear, this effect is used. In this commercial, the "Fruit of the Loom" men are a pop music group (with the "Apple" as the lead singer), and they're performing a ballad about a little boy who loves his "Fruit of the Loom" underwear.
[edit] Film based multiple exposures
Photographic film is sensitive to light in an integral way, i.e. functions like a true integral over time, and therefore "sifts out" the effect of a delta measure (flash).
[edit] 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.
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 light-vectoring. A more colloquial term for light-vectoring is dusting.
[edit] 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 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.
