Multiple flash is a photographic lighting technique in which a flash unit is fired two or more times during a single exposure, either to accumulate sufficient light output to achieve a desired exposure that a single flash discharge alone could not provide, or to create distinctive creative and scientific effects that exploit the stroboscopic nature of repeated flash bursts to record multiple stages of a movement or action within a single frame.
In its purely technical application, multiple flash is used to increase the effective light output available from a single flash unit, allowing a smaller aperture to be selected than would be possible with a single discharge alone. This approach is particularly useful in large spaces or when photographing subjects at greater distances where a single flash would provide insufficient illumination to achieve both correct exposure and the depth of field required by the subject. By firing the flash repeatedly during a single long exposure made in darkness or near darkness, the light from each successive discharge accumulates on the film or sensor, effectively multiplying the total light output available. The number of flashes required to achieve a given exposure at a specific aperture can be calculated from the flash unit's guide number, with the total accumulated exposure equivalent to the sum of all the individual flash contributions.
The creative applications of multiple flash are perhaps even more compelling than its purely technical uses. When a subject moves continuously during a single long exposure in a darkened environment and the flash is fired repeatedly at regular intervals during that movement, each flash discharge freezes the subject at a different stage of its motion and records it as a distinct, sharp image overlaid on the same frame. The resulting photograph shows a sequence of frozen positions of the moving subject superimposed against a dark background, creating a visual record of the complete arc or sequence of the movement that reveals its geometry, timing, and dynamics in a way that no single exposure could capture. This stroboscopic multiple flash technique has produced some of the most scientifically informative and visually striking images in the history of photography, made famous by the pioneering work of Dr Harold Edgerton at MIT, whose multiple flash photographs of subjects including golf swings, tennis strokes, drops of milk, and the flight of birds and insects opened up new visual worlds of motion analysis and scientific understanding.
Sporting subjects are among the most widely photographed applications of the creative stroboscopic multiple flash technique, with the swing of a golfer, the stroke of a tennis player, the kick of a footballer, or the movement of a gymnast all lending themselves naturally to the technique, producing images in which the complete arc of the movement is revealed as a graceful sequence of overlapping frozen positions that convey the full dynamic quality and athleticism of the action. Wildlife photography similarly benefits from the technique when applied to fast moving subjects such as the wing beats of birds in flight or the movement of nocturnal animals, where the rapid repetitive motion of wings or limbs can be decomposed into a revealing sequence of frozen stages by careful application of stroboscopic multiple flash.
Multiple flash can also be used in combination with camera movement or subject repositioning between flashes to create multiple exposure effects within a single frame, placing the same subject in several different positions across the image area and producing images in which a person or object appears to be present in two or more locations simultaneously. Careful attention to the tonal values of the background and the positioning of the subject is required to prevent the overlapping images from merging into an unreadable confusion of superimposed forms, and the technique generally works most effectively against a plain dark background that provides a clear separation between the multiple recorded positions of the subject.