How does film lighting work?

A camera needs light, but film lighting is not about making things bright. It is about shape. A face lit flat from the front looks like a passport photo. Light it from the side and the nose, cheeks and eyes suddenly have depth.

The classic recipe uses three lamps. The key light is the main one and makes the shadows. The fill light, softer and dimmer, lifts those shadows so they are not pitch black. The back light (or rim) sits behind the actor and draws a bright edge on the hair and shoulders, so they stand out from the background.

The lighting ratio compares the lit side of the face with the shadow side. With a key of 300 lux and a fill of 300 lux, the bright side gets 600 and the dark side 300: a 2:1 ratio, one stop, cheerful and open. Drop the fill to 43 lux and you get 8:1, three stops: moody and mysterious.

Where the key sits makes the famous patterns. Rembrandt lighting, named after the Dutch painter, leaves a small triangle of light on the shadow cheek. Loop, butterfly and split are the others. The camera, lens and exposure side is in CineCameraClear, and the whole film crew is in FilmClear.

Why is one shadow crisp and another one blurry? It depends on how big the light looks from the actor. A bare lamp is small, so every point of a shadow is either lit or not: hard light, with sharp edges. A big source lights each edge from many directions at once, so the shadow fades gently: soft light.

Film crews make a small lamp big in three ways. Shine it through a frame of white diffusion cloth. Put it in a softbox. Or bounce it off a white card or wall. Moving a soft source closer makes it look bigger, and even softer. The sun is huge but so far away that it looks tiny (half a degree), which is why sunlight is hard. Clouds turn the whole sky into one giant softbox.

Distance also changes brightness, very quickly. For a small lamp, double the distance and you get a quarter of the light, two stops darker. That is the inverse square law: light spreads over an area that grows with the square of the distance. Big soft sources bend the rule when they are close.

In India, cameraman Subrata Mitra is widely credited with pioneering bounce lighting on Satyajit Ray's films in the 1950s, bouncing lamps off white cloth to copy soft daylight.

White light is not all the same white. A candle is orange, a film studio lamp is yellowish, and daylight is bluish. Film crews describe these colours by colour temperature, in kelvin (K): the temperature a glowing piece of metal would need to glow that colour. Studio tungsten lamps are made for 3,200 K. Daylight for film is 5,600 K. Oddly, the “warm” colours have the lower numbers.

Your eyes quietly adjust to most light, so a room lit by bulbs still looks white to you. A camera does not guess. You set its white balance to tell it which light should look white. Set 3,200 K and tungsten looks white, but daylight looks blue. Set 5,600 K and daylight looks white, but tungsten looks orange.

To make lamps match, crews clip coloured plastic called gels in front of them. CTO (colour temperature orange) turns daylight into tungsten. CTB (colour temperature blue) turns tungsten into daylight, but it eats light: full CTB lets through only about a third. Gels are rated in mireds (a million divided by the kelvin), because one gel shifts every lamp by the same number of mireds.

Mixing colours on purpose is a classic look: a warm lamp inside a room, with cool blue light from the window. Modern LED lamps can dial in any kelvin, and even any colour, with a knob.

Before anyone speaks, the light has already told you what kind of film you are watching. High-key lighting is bright and even, with soft shadows and a light background: comedy, adverts, the family sitcom. Low-key lighting is mostly darkness, with one hard light and deep shadows: crime, mystery and 1940s film noir, with stripes of light through window blinds.

Break nature's rules and people feel uneasy. The sun and lamps shine from above, so light from below the face turns its shadows upside down: the horror look. A low golden-hour sun from behind feels warm, nostalgic and romantic. And moonlight in films is often really daylight: shot day for night, two stops dark and balanced blue.

Good cinematographers make light motivated: it should seem to come from something in the story, like a window, a table lamp in shot (a practical) or the moon. The audience may never notice the real lamps, because each one has an excuse. In India, V. K. Murthy's beam of real sunlight, bounced into a dark studio with two big mirrors for Kaagaz Ke Phool (1959), is a famous example of light telling the story.

A good lighting crew spends as much time taking light away as adding it. Barn doors, the four hinged flaps on a lamp, trim the edges of the beam. A flag is a black cloth on a stand that throws a shadow exactly where light is not wanted, such as on the wall behind an actor. A net or scrim is an open mesh that dims a light without softening it: a single net cuts about half a stop, a double about one stop.

Even a white wall is a light. It bounces soft light into the shadows. Put up a big black flag on the dark side instead, called negative fill, and the shadows get deeper and the face gets more shape.

The gaffer is the chief lighting technician: they choose and place the lamps and run the power. The key grip leads the grips, who rig the stands, flags, nets and frames. Together they build what the director of photography designs (see FilmClear).

Lamps have changed. A classic 1 kW tungsten Fresnel turns most of its power into heat. An HMI makes daylight-coloured light about four times more efficiently. Modern LED Fresnels sold as a match for a 1 kW tungsten lamp draw about 150 to 220 W, stay fairly cool and can dial any colour. A lamp you can see in shot, like a table lamp, is called a practical.

Now you are the director of photography. The scene: three people at a dinner table. The director wants a mood, and you have four lights. A key Fresnel you can move round the table, a soft fill by the camera, a back light behind the far chair, and a practical pendant lamp hanging over the table, which also motivates the rest.

The hard part is lighting three faces at once. A key that shapes one face beautifully may leave another in flat front light, or in total darkness. Every face has to read: bright enough to see the eyes, not so bright that it glows. The readout meters each face like a DoP would, and checks the lighting ratio.

The histogram counts the pixels in the camera view from black on the left to white on the right. A high-key scene piles up on the right, a noir scene on the left. Cinematographers also watch it so nothing important clips to pure white or sinks into pure black.