Lighting Ratios | Background Light Intensity
Subject-to-Background Distance

Lighting Ratios

Unless each of the four lights we've discussed is at the proper intensity, the formula lighting approach will not work.

Since the key light is the dominant light on the subject, it must be stronger than the fill light. In color production the fill should be about one-half the intensity of the key.

This key-to-fill brightness difference is expressed in terms of a lighting ratio.

If the key light is twice as bright as the fill, the ratio will be 2:1 (which is the standard for color TV). Using the 2:1 ratio, if the key light is 2000 lux, the fill will be 1000 lux; if the key light is 90 foot-candles (FC) the fill light would be 45 FC.

Although many lights may be used in a scene, the lighting ratio refers to the ratio between just two lights: key and the fill.

The key-to-fill ratio affects how the form, dimension and surface texture of subject matter will be seen. To achieve dramatic effects, and occasionally to meet the needs of special subject matter, ratios other than 2:1 can be used. More on that later.

If a lux or foot-candle meter isn't available to establish the proper lighting ratios, a standard photographic light meter can be used. The f-stop difference between the intensity of lights can be translated into a lighting ratio.

To achieve a standard 1:2 ratio, make the key light one f-stop brighter than the fill. The key light in this example could (when measured by itself) call for an exposure of f:16 and the fill light (by itself) an exposure of f:11.

Lighting Ratios

With Differences (In F-stops) Required Between Key and Fill Light Intensities.

1:1 - no difference (flat lighting)

2:1 - One f-stop (for general color photography, videography)

3:1 - One and two-thirds f-stops (for general black and white photography, videography)

4:1 - Two f-stops (for low-key dramatic effect)

8:1 - Three f-stops (for a very low-key dramatic effect) For ratios beyond this the dark areas will probably just be rendered as black, without discernible detail.

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Background Light Intensity

Because the background is of secondary importance to the center of interest, it should receive a lower level of illumination. Generally, the intensity of background lights should be about 2/3 the intensity of key lights. This will insure that the central subject matter stands out slightly from the background.

In case you have forgotten Math 101, you can get two-thirds of any number by multiplying it by two and dividing the result by three. Therefore, if the key is 2000 lux, the light falling on the background should measure about 1300 lux.

If you are using a photographic meter to set light intensities, the background light should read 1/2 to 2/3 of a stop less on the exposure meter than the key light.

Since backgrounds are typically one-dimensional (flat) and of secondary importance to the main subject matter, the placement of the lights and their angles is not critical. But, the light across the background should be even--especially if you are using chroma key. By walking along the background with a light meter, any dark or bright areas can be quickly found.

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Subject-to-Background Distance

Shadows on backgrounds from mic booms, moving talent, etc., can be distracting and annoying. Background lights will lighten, but normally not eliminate, shadows. However, by moving subjects 3 meters (9 or more feet) away a background, you will find (if the key is at an elevation of 45 degrees) that shadows will end up on the floor (out of sight) instead of on the back wall behind the subject.

Sometimes, however, it's necessary for talent to move in close to a background. An example would be someone shadows from the front lights almost invisible--if you don't mind the soft, diffused look it will create in the video. Otherwise, you will just need to use a key angle that doesn't create distracting shadows.

Unduly dark backgrounds can be brightened up by using explaining a chart on a wall.

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