Key light in the context of Fill light

Low-key lighting is a style of lighting for photography, film or television. It is a necessary element in creating a chiaroscuro effect. Traditional photographic lighting (three-point lighting) uses a key light, a fill light and a back light for illumination. Low-key lighting often uses only a key light, optionally controlled with a fill light or a simple reflector.

Low key light accentuates the contours of the subject by throwing areas into shade while a fill light or reflector may illuminate the shadow areas to control contrast. The relative strength of key-to-fill, known as the lighting ratio, can be measured using a light meter. Low key lighting has a higher lighting ratio, e.g., 8:1, than high-key lighting, which can approach 1:1.

Low-key photography is a genre of photography consisting of shooting dark-colored scenes by lowering or dimming the "key" or front light illuminating the scene (low-key lighting), and emphasizing natural or artificial light only on specific areas in the frame. This photographic style is usually used to create a mysterious atmosphere, that only suggests various shapes, often graphic, letting the viewer experience the photograph through subjective interpretation and often implies painting objects or the human body with black non-toxic dyes or pigments.

Renaissance and Baroque, represented by different painting styles including sfumato and chiaroscuro used by artists like Leonardo da Vinci and Rubens), tenebroso (it. dark, mysterious) used by artists such as Caravaggio, Rembrandt, Jusepe de Ribera among others, produced paintings in which black was predominant on the canvas and the light often come from only one source to achieve dramatic scenes.

Lighting ratio in photography refers to the comparison of key light (the main source of light from which shadows fall) to the total fill light (the light that fills in the shadow areas). The higher the lighting ratio, the higher the contrast of the image; the lower the ratio, the lower the contrast. The lighting ratio is the ratio of the light levels on the brightest-lit to the least-lit parts of the subject; the brightest-lit areas are lit by both key (K) and fill (F). The American Society of Cinematographers (ASC) defines lighting ratio as (key+fill):fill, or (key+Σ_fill):Σ_fill, where Σ_fill is the sum of all fill lights.

Light can be measured in footcandles. A key light of 200 footcandles and fill light of 100 footcandles have a 3:1 ratio (a ratio of three to one) — (200 + 100):100.

High-key lighting is a style of lighting for film, television, or photography that aims to reduce the lighting ratio present in the scene. This was originally done partly for technological reasons, since early film and television did not deal well with high contrast ratios, but now is used to suggest an upbeat mood. It is often used in works of comedy. High-key lighting is usually quite homogeneous and free from dark shadows. The terminology comes from the higher balance in the ratio between the key light and the fill light in a traditional three-point lighting setup.

In the 1950s and 1960s, high-key lighting was achieved through multiple light sources lighting a scene—usually using three fixtures per person (left, right, and central)—which resulted in a uniform lighting pattern with very little modeling. Nowadays, multiple hot light sources are replaced with much more efficient fluorescent or LED soft lights, which provide a similar effect.