Light Falloff Inverse Square Calculator
Light from a small source spreads out as it travels, so the further a subject sits from a lamp the dimmer it is, and the relationship is the inverse-square law: brightness falls with the square of distance. This calculator takes a starting brightness and distance and a new distance and returns the new relative brightness, the ratio compared with the start, and the change in exposure stops. Photographers use it to set light-to-subject distance for the falloff they want, and to keep exposure correct when a light is moved.
Inverse-square law formula
New brightness E2 = E1 * (d1 / d2)^2
Brightness ratio = (d1 / d2)^2
Exposure change (stops) = log2(ratio) = 2 * log2(d1 / d2)
A positive stop value means brighter, negative means dimmer
Only the ratio of distances matters, so any consistent unit works. Doubling the distance gives one quarter the light, a drop of 2 stops.
Lighting falloff context
- Double the distance and brightness falls to one quarter (2 stops down).
- A close light has rapid falloff, darkening the background for depth.
- A distant light gives gentle, even falloff across subject and background.
- The law holds for point-like sources; large softboxes only at several times their width.
- Use the same distance unit for both inputs; the result is unit-independent.
Light falloff: frequently asked questions
What is the inverse-square law for light?
Light from a point source spreads over a sphere, so its intensity falls with the square of distance. Intensity at the new distance equals the original intensity times the original distance squared divided by the new distance squared: E2 = E1 times (d1 / d2) squared. Double the distance and brightness drops to one quarter.
How many stops do I lose by moving a light back?
The change in stops equals the base-2 logarithm of the intensity ratio, which is 2 times the base-2 log of (start distance / end distance). Moving from 2 metres to 4 metres is one doubling of distance, a drop of 2 stops, leaving one quarter of the light.
Does the inverse-square law apply to all lights?
It applies to point or small sources where the distance is large compared with the source size. Large softboxes and the sun behave differently up close: a softbox acts more like a point source only at several times its own width, and the sun is effectively at constant distance for any earthbound subject.
How can I use falloff to control a portrait?
Place a light close to the subject and the brightness drops off quickly behind them, darkening the background and adding depth. Move the light far away and the falloff across the scene is gentle, lighting subject and background more evenly. Distance is a creative control, not just an exposure number.
What units should I use for distance?
Any unit works as long as both distances use the same one, because only the ratio matters. Enter both in metres or both in feet. The intensity ratio and stop change are dimensionless, so the result is identical either way.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.