A key light illuminates the subject. It creates bright highlights and deep shadows. The shadows may be too dark. The fill light softens them. It is a secondary light that fills in the shadows created by the key light. It does not eliminate the shadows. It reduces their intensity. The ratio of key to fill determines the contrast of the image. A high ratio creates dramatic, moody lighting. A low ratio creates flat, even lighting.

The fill light is usually softer and less intense than the key light. It may be a diffused lamp, a bounce board, or a reflector. It is positioned opposite the key light, often at a lower angle. The fill light should not create its own shadows. It should be invisible, just lifting the dark areas enough to reveal detail. The amount of fill is a creative choice. A noir film may use almost no fill, creating deep shadows. A sitcom may use heavy fill, creating a bright, even look.

The fill light is part of the three-point lighting setup, along with the key light and the backlight. The key light provides the main illumination. The fill light softens the shadows. The backlight separates the subject from the background. The three lights work together to create a controlled, dimensional image. The setup is a starting point. Cinematographers adjust it for every scene.