Click to explore the fill light.

In addition to the main light, many photos are taken with a fill light. This light represents the light that falls on an outdoor subject from the broad expanse of an open sky, or reflecting from surfaces in the landscape. This light, placed opposite the main light, opens shadows by lighting the dark side of the subject facing away from the main light. Its intent is to lighten the shadows to reveal otherwise hidden detail, not eliminate the shadows. In some situations, a fill card is used instead of the fill light. It reflects light from the main light back onto the dark side of the subject. You can change the position of the fill light for different effects and to eliminate hot spots. Also, when placing it closer to the lens axis you reduce the possibility of creating a confusing or unflattering second set of crisscrossing shadows on the subject.

Since fill light has less intensity than the main light, it can more easily be overwhelmed by ambient light coming from room lights or windows. This ambient light isn't necessarily bad, provided you are aware of the effects it is having on the light and white balance. To eliminate it, you can turn off the room lights and use window shades or blinds to block or reduce daylight streaming through the windows.

The fill light is almost always less bright than the main light, in fact about half as bright. Its relative brightness can be controlled in a number of ways. For example, it can be placed farther away, a diffuser or scrims can be used, or you can use a less powerful light. On strobes you also adjust lighting ratios to make one light brighter than the other by a specified number of stops. Some continuous lights are equipped with a dimmer switch.

When using two or more light sources on a subject, they can be of varying intensities either because of their differences in brightness or their differences in distance from the subject. The light they throw on the subject can be expressed as a ratio. For example, if two lights illuminate the subject with the same intensity, the ratio is 1:1. As one source gets brighter, the ratio can change to 2:1, 3:1, 4:1, and so on. The first number in the ratio is the number of stops difference between the two levels of illumination. The higher the ratio, the more contrast the image will have. At very high ratios, white will be burned out or shadow areas will be black.