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Concept
Postlumination
Résumé
In cinematography and photography, flashing is the exposure of the film or digital sensors to uniform light prior to exposing them to the scene. It is used as a method of contrast control to bring out detail in darker areas. This adds a bias to the overall light input recorded by the sensor. When used for artistic effects, it can be used to add a colour cast to shadows without significantly affecting highlights. Flashing is usually described as a percentage of exposure increase to the film's base fog level. While the flash itself is often a neutral color temperature, the flash exposure could be any color: the color of the flash will be imbued disproportionately into the shadows of the image.
The effect is produced by adding a small and even level of exposure to the entire image. Since exposure levels increase logarithmically, this tiny level of additional exposure has no practical effect on an image's mid-tones or highlights, while it shifts the darker areas of the image into the practical sensitivity range, thus allowing the darker areas of the image to show visual detail rather than uniform blackness.
Flashing can be applied to the film stock before, during, or after the principal photography of a motion picture, although always before the film is processed. When applied before or after shooting, this is generally done by a film laboratory. The level of flashing needs to be tested beforehand and subsequently moderated appropriately against the light levels of the scene, or else it risks having a minimal impact if too low or making the shadows "milky" when too high.
Adding a general overall exposure of light to a photosensitive material to alter the material's response to a captured image is a long-known technique. Photographer Ansel Adams describes the use of "pre-exposure," to make details visible in a darker area of an image, in his text The Negative (rev. ed. 1959). For more, study astronomic photographic techniques when silver-halide emulsions on glass plates were the available tools.
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