Unless each of the four basic lights we've discussed is at the proper intensity, the formula lighting approach — or any good 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 most TV applications. At the same time, as we've noted, some lighting directors, especially in TV news, prefer to make the key and fill the same intensity, resulting in a ▲flat, high-key effect. This option will be discussed more fully later.
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: the key and the fill.
The key-to-fill ratio affects how the form, dimension, and surface texture of subject matter will be rendered. To achieve dramatic effects, and occasionally to meet the needs of special subject matter, ratios other than 2:1 can be used.
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 2:1 ratio, for example, we assume that a light that (by itself) calls for an exposure of f:16 on a meter is twice as bright as one that registers f:11. Using this principle we can set up our key and fill lights according to the lighting ratios below.
With differences (in f-stops) required
Recall that a simple way of establishing lighting ratios is by controlling the distances between the lights and the subject.
Sometimes it's desirable to minimize or smooth out the surface detail. If highly diffused key and fill lights are used close to the camera surface detail and apparent depth will be minimized.
Reducing the key-to-fill lighting ratio to 1:1, with the key intensity equal to the fill intensity, adds to this flat lighting effect. We'll re-visit our jewelry box to illustrate this. The first photo below was shot with a low lighting ratio (flat lighting), the second goes to the other extreme with a high key-to-fill lighting ratio.
Although form and dimension are sacrificed in flat lighting, this type of lighting can be useful in minimizing wrinkles and skin problems, and in creating a soft, flattering effect for the human face. This could be very important in a cosmetic commercial, for example.
In contrast, by increasing the key-to-fill ratio to 1:5 and beyond, surface detail and texture will be emphasized—especially if a hard key light is used at an angle from 65 to 85 degrees off to one side, as shown on the right above.
The same hard-soft lighting differences are present outside. The two photos below were taken of the same section of concrete blocks on a wall. The photo on the left was taken on an overcast day, and the photo right was illuminated by the overhead sun on a clear day. Here we can see the difference in both the quality (hardness and softness) of the sunlight and the lighting ratio. On the overcast day the key-fill ratio ends up being about the same, because the light is diffused.
Back Light Intensity *
To provide the subtle rim of light around subjects the back light has to be slightly brighter than the key. In the case of an on-camera person, back light intensity will depend on the hair color and clothes.
Subjects who have brown hair and clothes in the mid-gray range will require a back light one and one-half times the intensity of the key. Assuming a key light intensity of 1,500 lux, the back light would then be 2,250 lux.
If you don't have a meter that reads in lux or foot-candles, you can simply move the back light slightly closer to the subject than the key light (with the key and fill lights on), until you see the desired subtle rim of light around the subject.
A person with dark hair and clothes will take more back light than a blond wearing light clothing. Be careful to observe the effect on a monitor or in a well-adjusted camera viewfinder.
With subjects who have hair and clothing of similar reflectance, the intensity of the back light is not too difficult to determine. But difficulties arise when a person has dark hair and a light coat, or blond hair and dark clothing. In such cases the beam of the back light(s) can be partially masked off with barn doors so that the brightest part of the beam will hit the dark areas.
The color temperature of the back light is not nearly as critical as it is with key and fill lights. Within limits, dimmers can be used.
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 light should be about 2/3 the intensity of key light. This will insure that the central subject matter stands out slightly from the background.
In case you've 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 2,000 lux, the light falling on the background should measure about 1,300 lux.
If you are using a photographic meter, you can set the background light 1/2 to 2/3 of an f-stop less 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 visual effects such as chroma key. By walking across the background area with an incident light meter, you can find dark or bright spots.
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 from 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 explaining a chart on a wall. The use of a large softlight would render the shadows from the front lights almost invisible — if you don't mind the soft, diffused look it will create. 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 a higher level of illumination, and bright, intrusive backgrounds can be "pulled down" by lowering background illumination.
Multiple Purpose Lights
Occasionally, you can make lights serve dual purposes and still maintain the three-point lighting effect. Here, a one-on-one interview is lit with three lights instead of six. Note that each of the (very carefully placed) lights serves two purposes.
If distances are carefully controlled, the lights will be 50 percent brighter as back lights than as keys.
This can work well under carefully controlled situations where you know in advance the color of each person's hair (or, possibly, lack of it) and the color of their clothes.
In using this approach you won't have much latitude in accommodating special needs. For example, the chairs can't be moved without upsetting the lighting balance.
Now that we've covered the basics of lighting, in the next module we'll take up some special lighting situations.
*Back light can be one or two words, depending on the context.
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