On a film set, the key light is the first light the gaffer positions. It is the primary source of illumination on the subject. It defines the shape of the face, the direction of the shadows, and the overall mood of the scene. A hard key light creates sharp, dramatic shadows. A soft key light creates gentle, flattering transitions. The choice of key light is the foundation of the lighting design.

The key light is usually positioned at a 45-degree angle from the camera and slightly above the subject's eye level. That placement creates a natural-looking shadow on one side of the face, giving the image depth and dimension. The exact position varies. A higher key light creates deeper shadows and a more dramatic look. A lower key light creates a more theatrical or ominous look. A frontal key light creates a flat, even look with minimal shadows.

The key light is part of the three-point lighting setup, along with the fill light and the backlight. The key provides the main illumination. The fill softens the shadows. The backlight separates the subject from the background. The ratio of key to fill determines the contrast. A high ratio creates a moody, low-key image. A low ratio creates a bright, high-key image. The key light is the starting point. Everything else is built around it.