Fill-in Flash; Fill-in Light; Fill; Fill Light
The term ‘fill-in’ or ‘fill’ is used in photography to indicate the use of additional lighting to brighten shadow areas or darker areas in a scene.
In very bright light or direct sunlight the range of contrast from black to white is too large for the digital image sensor to resolve. Dark areas become black and lose detail. ‘Fill-in’ light is used to brighten the darker areas so the camera can capture the detail that would have been lost in pure black.
Fill-in light can be used to soften the shadows from a main hard light, or to brighten enough for the dark area to be clearly visible on the picture.
The fill-in light must be of a lower intensity than the main light or it would make the shot look unbalanced or unnatural. Usually, camera pop-up flash, off-camera flash, studio strobe, soft boxes or other lighting is used. However, reflectors, remote light sources and even hand-held torches will all provide a source strong enough to soften shadows.
When using flash or studio strobes the term applied is Fill-in Flash. The use of always-on or continuous lights or diffusers requires the use of the term Fill-in Light.