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The key light is the first and usually most important light that a photographer or cinematographer will use in a lighting setup. The purpose of the key light is to highlight the form and dimension of the subject.

The key light could be hard or soft and depending on the desired setup can be placed at a different angle. When part of the most common setup, the three-point lighting setup, the key light is placed at a 30-60° angle (with the camera marking 0 degrees). In addition to the horizontal angle, the key light could be high or low producing different effects. The most common vertical position for the key light is at a 30° degree angle (i.e. slightly above the eye line, the nose shadow should not cover the lips).

A key light positioned very low will distort the actor's features, an effect that can be used successfully in a horror or comedy: imagine a flashlight illuminating the face from below. A very high key light will result in more articulated cheek bones and long nose shadows. Marlene Dietrich was famous for demanding that her key light be placed high.

Using just a key light is usually unacceptable as the contrast between the light and dark will be too great. A fill light is usually used to decrease contrast and add more details to the dark areas of an image. An alternative to the fill light is using a light reflector (even a white wall could do).

The key light is not a rigid requirement. When the subject is moving or turning around, the key light can become a back light and the back light may become the key light.