light
‘Fall-off’ and the Inverse Square Law
The way that light decreases as you move away from the light source is expressed scientifi cally by
the Inverse Square Law, which states:
‘When a surface is illuminated by a point source of light, the intensity of the light at the surface is
inversely proportional to the square of the distance from the light source’.Now, just before you panic this is really quite simple. It means that if your subject moves away from
your camera’s fl ash, there is a fall-off in the light that they receive. In fact, if they are now twice
as far away, they do not receive half as much light but one quarter. Four times the distance, one
sixteenth of the light! This does not happen in sunlight because we are all the same distance from
that big point source in the sky, the sun. But it is essential that the Inverse Square Law is applied
when dealing with fl ash, other artifi cial lights, refl ected light or window light as the visual effect of
subjects at differing distances to the light source is uneven illumination.
ACTIVITY 2
Using a fl oodlight or window light, illuminate two people standing at three feet and six feet from
the light source.
Position the people six feet further away from the light source so that the people are now nine
and 12 feet from the light source.
Does the difference in brightness between the two people illuminated by the light source increase
or diminish as the light source moves further away from the subjects?Orien Harvey