Fill-in flash, or fill flash, is the use of a flash as a secondary light that adds to the main light, usually daylight. It brightens shadows without replacing the existing light.
It is very useful when the subject is backlit or in strong sun, which causes dark shadows on faces. Most cameras with a built-in flash offer a fill flash setting, and it also works with external flash units. Fill flash can also make colors and contrast on a nearby subject look better when the light is dull.
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