Digital SLR cameras can be used with powerful flash units mounted on the camera accessory
shoe. Such optional flash units can illuminate distant subjects for optimal exposure.
Optional flash units can be used not only when natural lighting is inadequate but also to "fill in" (illuminate) shadows and backlit subjects or to add a catch light to your subject's eyes.
If the camera is not equipped with a built-in flash, an optional flash unit can be mounted on the accessory shoe for flash photography.