How Distance Affects

Foot-Candle Intensity

Updated: 12/05/2008

Light measured on an object is inversely proportional to the distance the object is from the light source. Thus, doubling the distance that a light is from a subject results in one-quarter the number of foot-candles of light on a subject. (Remember, you can multiply foot-candles by 10 to get the approximate light levels in lux.)

The chart on the left shows the relationship between distance and foot-candles for studio LED and fluorescent lighting instruments.

The chart on the right shows how intensity diminishes when a 1,000-watt studio Fresnel is used in the flood and spot positions. The lamp distance, inverse-square law does not strictly hold when reflectors are used in lighting instruments such as Fresnels.

Even so, the relationship between light distance is important to know (especially on locations where dimmers are not commonly used) and lighting ratios between the various lights are being established. As you will note from these charts, a few feet of added distance will significantly diminish the brightness of light on a subject.