Standard lighting in film uses three light sources: key, back and fill. Key light, in this context, is the primary light source that allows the viewer to see the person, object or scene. Back lighting is placed behind the object to allow viewers to perceive the depth of a scene, and fill lighting is usually a soft light that reduces shadows caused by the key light. In low-key lighting, only one light source is used, but unlike the key light in standard lighting, which is generally placed in front of and above the subject, low-key lighting may be placed anywhere, including above, below or behind the subject.
Low-key lighting, in both film and photography, is used to create images with high contrast between dark and light areas of the film. This effect, often known as chiaroscuro, uses deep shadows to emphasize the shape of an object and reduce or even eliminate the background. These shadows may be used to create a tone of suspense by implying through sound or scripting that something the viewer cannot see is nearby. Or it can create a more brooding atmosphere, in which the shadows are less ominous, but obscure the details of facial expressions or gestures.
Not all low-key lighting comes from a single source; sometimes fill lights are used to add visibility while maintaining the high contrast effect of low-key lighting. In this case, a ratio of one to eight is used to describe the fill to key light balance, meaning that the fill should be no more than 1/8 as strong as the key light.
The angle at which the light hits the object controls what is hidden by shadow and what is revealed. In low-key lighting, this is extremely important because the shadowed parts can be completely hidden. Using a single light from behind an object creates a silhouette and can give the effect of a halo around the object. Lighting from one side emphasizes the curves and angles of the subject's shape, and can be used to create a sultry look, especially when the subject is a person. Low-angle front lighting is used on human subjects for creepy effects because the cheekbones shadow the eyes, while high-angle lighting from in front of and above the object creates a more natural look with the added contrast of light and shadow.
Low-key lighting can be hard or soft, meaning the light source can have a sharp edge or be diffused. Soft (diffused) lighting will have slightly lower contrast and a softer blend between the bright and shadowed parts of the image, while hard (sharp-edged) lighting will blend more quickly from bright to dark, creating a starker image.