Fill flash is a photography technique that uses a flash to fill the darker shadows of a subject in a photo. It prevents the subject from being too dark while the background is perfectly exposed. You should use fill flash in situations when there is more light behind the subject rather than on the subject and when there is an unwanted shadow on the subject. In portraits, fill flash helps get rid of the unwanted shadows, but you have to make sure the light still looks natural. You want the background to be perfectly exposed and the subject to be slightly underexposed so your flash doesn’t make the subject too bright. Fill flash can also be useful in macro and landscape photography.