Lighting · Inverse-square law
Light distance: double it and the actor gets a quarter of the light. That is two stops.
A teaching model of a small (point) source: illuminance = output at 1 m ÷ distance², with the background directly behind the actor on the light's axis and both surfaces the same tone. Soft boxes, fresnels, windows and other large sources fall off more gently than this at close range (within a few times their own size), so read real lights with a meter. Fixture outputs are rough examples. Camera stop uses an incident-meter constant of 250. 1 fc = 10.76 lux.