As a light source is moved further from a subject its intensity reduces by the square of its inverse. If a light is moved twice the distance from a subject the amount of light on the subject will be the inverse of 2 2 (the doubled distance). This is (1/2 2) or one quarter. If the distance is quadrupled it will be (1/4 2) or a 16th of the intensity, at eight times the distance (1/8 2) one-64th of the intensity. An object 1m from a small light source will receive four times as much light as an object twice as far away.
There will be two stops difference in the exposure reading from nearest to farthest. Quadruple the distance and there will be a 16th of the light which is four stops less light.
The spread of light can be compared to the spread of paint from the nozzle of a spray can. A surface held very close to the spray will receive an intense stream of paint focused in a small area. If the surface is moved back twice as far the spray will cover an area four times greater, but the density of the paint will be four times less (for the same duration of spray).
This dramatic ‘fall off’ is only significant when a light source is close to the subject. The sun is 93 million miles away so the difference of distribution across the earth is imperceptible. However a lamp 2 m from an object of 2 m width will illuminate the closest edge four times as brightly as the far side.
Equally, across a row of people at the same distance the nearest person will be two stops brighter than the farthest. (To make the light distribution more even the light must be either moved away from the subject so that the difference across is less significant – or the light can be bounced, so that the increased area of light will reduce the distance to the overall subject.)
