The inverse square law says that the light from a small source falls off with the square of the distance. If you double the distance between the light and the subject, the subject receives only one-quarter of the light.
This matters most with flash and studio lights. For example, if you move a light from 2 meters to 4 meters away, the light on the subject drops by two stops. To keep the same exposure, you need four times more light or two stops more exposure, for example by opening the aperture two stops. See also guide number.
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