what is fill flash? Fill flash is a technique photographers use to brighten deep shadowed areas, especially on gray days or extremely sunny sessions. When the subject is underexposed due to the deficit of sunlight and other lights or sometimes due to the backlight, fill flash is supplemented to correctly expose the subject. When it comes to portraits, fill flash can spice up them by ironing out the unwanted shadows and dark circles under the eyes of your subject. You  don’t want your subject to look unrealistic due to a superfluous light.