Exposure / Light
Inverse-Square Light Falloff Calculator
Find how many stops of light you gain or lose when you move a light source, and the equivalent aperture to compensate, live as you type.
Distances
Use any consistent unit (feet or meters) for both distances — only the ratio matters.
Reference
Light intensity falls off with the square of distance from the source. Doubling the distance from a light drops intensity to a quarter (2 stops); moving to 1.4× the distance costs exactly 1 stop.
Enter your current aperture to see the equivalent f-stop needed at the new distance to maintain the same exposure.
Results
Why Moving a Light Costs More Than It Looks Like
Light doesn’t fall off in a straight line as it travels — it drops with the square of the distance from the source. That single fact explains why pulling a key light back “just a little” can cost a full stop, and why light near a subject falls off fast while light far from a subject stays comparatively even across a scene. This tool converts any change in light-to-subject distance directly into stops.
The Inverse-Square Law
Double the distance from a point light source and intensity drops to a quarter — a 2-stop loss. Move to 1.4× the original distance and you lose exactly 1 stop. This relationship holds for any point or small light source; large modifiers like softboxes fall off more gradually at close range but converge toward the same inverse-square behavior farther away.
Compensating With Aperture
Enter your current aperture and the tool returns the exact f-stop you’d need at the new distance to keep the same exposure — handy when you need to move a light for spacing or safety reasons but can’t afford to lose the look you’ve already dialed in.
This tool is especially useful for:
- Gaffers and lighting techs repositioning a light without losing the planned exposure.
- Photographers pre-planning lighting ratios across multiple subject distances.
- Anyone learning how the inverse-square law actually behaves on set.