Inverse Square Law
The rule that light from a point source falls off in proportion to the square of the distance, so doubling the distance quarters the illumination.
Formula / rule
E ∝ 1 / d²
Measured in
stops (EV)
In depth
This single relationship governs almost every lighting decision in a studio. Because the falloff is steepest close to the source, a light placed near the subject produces rapid, dramatic gradation across a face, while the same light moved far back produces flat, even coverage. Moving a light from one metre to two metres costs two stops, not one.
Examples in practice
A softbox at 1 m metering f/11 will meter f/5.6 at 2 m and f/2.8 at 4 m.