Slave flash units provide additional light for photographs.
In photography, good lighting is essential. Even if you have a sophisticated flash unit mounted on your camera, it is sometimes helpful to employ "slave" flash units, which supplement the lighting that the main or "master" unit provides.
Function
A slave flash fires when it detects the light from the master flash. Because light travels so quickly, for all practical purposes, the flashes fire simultaneously.
Lighting From Multiple Angles
There are times when a photographer wants to light a subject from more than one angle to reduce shadows or highlight some features. A slave flash can be a nice alternative to expensive studio lighting for this purpose.
Filling in the Shadows
Slave flashes can also provide an easy way to light multiple locations in a scene. For example, if you were photographing a living room, you might aim a slave flash at a dark corner that light from the main flash does not reach.
Disadvantage of Slave Flashes
Slave flashes can be a handy alternative to expensive studio lighting. However, studio lighting is constant, making it easier to gauge whether you have the correct exposure before you take the photo. With slave flashes firing only at the moment the photo is taken, some trial and error may be necessary to get the lighting just right.
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