Key Light

The main light on a person. It is the brightest in the scene and decides which way the light comes from and where shadows fall. It is usually placed 45° to the side of the camera but can go anywhere the scene needs, and where it sits changes the name of the face lighting: loop, Rembrandt, split and so on. A soft, warm key feels friendly; a single hard light feels dramatic.

Lighting positions seen from above. Only one key light, 45° to the camera’s left and a little above eye level.

Camera view. The lit side of the face is bright, and the other side sinks into shadow, blending into the dark background.

How it feels

Depth
Dramatic
Contrast
Focus

When to use it

The starting point for lighting any person
Creating a dramatic mood with a single light
Making light look like it comes from a window or lamp

In the movies

Raiders of the Lost Ark (1981) · Source: Rotten Tomatoes Coming Soon (YouTube) · Original video

Lost in Translation (2003) · Source: Movieclips (YouTube) · Original video

Not to be confused with

Fill Light: Fill is a supporting light that softens the shadows the key makes, and it is dimmer than the key.
Three-point Lighting: Basic lighting that adds fill and back lights to the key. Shadows soften and the outline separates from the background.
High-key: It has "key" in its name, but it means the brightness and contrast of the whole image, not the height of the key light.

Prompt

Keyword

key light

Descriptive

A single soft key light from camera left at 45 degrees, slightly above eye level.

Try these combinations

Key light + close-up: a face made with one light
Close-up of an old sailor, warm 3000K main light from left, deep shadows, dark background
Key light: light from an overhead lamp
Medium shot of a woman reading at night, warm key from overhead practical, cozy room