The inverse-square law, explained with a softbox and a stool

28 October 2026 · ShotBox3D team · Exposure, Fundamentals

Why moving your key light from 1 m to 2 m costs you two full stops, and how to use that fall-off to control the background without touching a dial.

Light spreads out as it travels. Double the distance from a light and the same amount of light covers four times the area, so each part of it gets a quarter as much. In photography terms, a quarter is two stops.

What it means on set

• Key at 1 m, model reads f/8. Move the key to 2 m and the same power gives f/4: two stops darker.
• Move it to 1.4 m (the square root of 2) and you lose exactly one stop.
• The background 3 m behind a model lit from 1 m gets about 4 stops less light than the face, so it falls to near black.

That last point is the useful one. If you want a dark background without a black backdrop, bring the light close and push the model away from the wall. If you want the background to stay bright, move the light further back: the face and the wall are then at more similar distances, so the fall-off between them is smaller.

Try it in the studio

Place one softbox, a model and a grey backdrop. Watch the meter's reading at the face while you drag the light back, then compare the backdrop's brightness in the camera preview. The ratio between them is the inverse-square law at work.