Inverse square law calculator
See how exposure changes when you move a light. Enter where the light is and where you want to move it; the calculator gives the change in stops, the light on the subject as a percentage, and the distances for one stop brighter or darker.
Any unit works, as long as both distances use the same one.
How to use it
- Light is now at: its distance from the subject.
- Move it to: the new distance.
Example
Moving a light from 2 m to 4 m cuts the light on the subject to 25%, a loss of 2 stops. One stop brighter is at about 1.41 m; one stop darker is at about 2.83 m.
Good to know
Light falls with the square of distance, so one stop is a factor of √2 (about 1.41) in distance. This holds for small, hard sources such as fresnels and bare bulbs; large soft sources fall off more slowly up close.