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Light intensity (quantity) is measured in foot-candles (candela) in the United States, or in lux in most other countries. Even in the United States lux seems to be replacing foot-candles. As we've noted, a foot-candle equals about 10.74 lux (or, for a rough conversion, multiply foot-candles by 10 to get lux).
Although most TV cameras need at least 1000 lux (about 90 FC) of light to produce good quality video in the middle of the lens f-stop range, many can produce acceptable pictures under a few foot-candles of light.
When color was first introduced in TV studios, 300 foot-candles (more than 3,000 lux) of light were required. As newer color cameras were introduced, this level kept dropping.
Today, many on-location shoots are done with as little as 30 foot-candles (about 300 lux) of light. The latest generation of professional video cameras can produce good quality video under less than one foot-candle (less than 10 lux) of light.
Part of the motivation for using less light on locations lies in limiting depth of field and creating a "film look" by using wide f-stops. Some DPs (directors of photography) keep their on-location camera lenses near their widest f-stop to maximize this effect.
Whereas in the studio you may want to use a mid-range f-stop to keep foregrounds and backgrounds in focus, on location and in dramatic production a selective focus effect is often preferred — especially in close-ups and medium close-ups.
Light metersare used to measure light intensity. As we will see, being able to establish rather exact intensities for the various lights is important for professional video work.
Uneven light around a scene will cause variations in video levels and even dark or washed out skin tones.
Because the eye is a rather unreliable judge in setting up lighting, you need to use either a light meter or a keen eye coupled with a high-quality color monitor. Although the latter is preferred for making final adjustments, when lights are first being put into place it's much faster to use a light meter.
A reflected light meter measures the amount of light being reflected from (off of) subject matter. This is the type of built-in light measurement system used in most still cameras.
Because a reflected light meter assumes that all subject matter reflects 18-percent of the light falling on it — a so-called average scene — it can be easily fooled by nonstandard subject matter. This is why the auto-iris or auto-exposure devices in video cameras can get you into trouble.
As the distance between a light source and the subject increases, the light is spread out over a larger area and the intensity decreases.
For example, when a light is 3 meters (10 feet) away from a subject you find that there's 4000 lux of light on your subject. If you then double the light-to-subject distance to 6 meters (20 feet), you will end up with only about 1/4 the original light, or 1000 lux.
This distance-intensity concept proves particularly useful in setting up lights on locations. In these situations altering light intensities becomes a matter of just moving the lighting stands closer or farther away from subjects.
Lastly, brightness can be reduced in incandescent lights by reducing the voltage to the lamps with dimmers. Unfortunately, this also affects color temperature.
A rough rule of thumb is that for every one-volt drop in the voltage to an incandescent light, the color temperature drops by 10K.
Since the human eye can detect a 200K color shift in the 2,000-4,000K range, this means that a studio light can only be dimmed by about 20 percent (in relation to the other lights) without being noticed.