Inverse square law

Inverse square law, in photography, applies to lighting. It applies to any sort of lighting really but its most relevant application is with off camera lighting.The inverse-square law teaches us how light works over distance and why the distance between your light source and your subject is so important.To get a correct exposure (assuming we use a consistent shutter speed), if the subject was very close to the light then we would need to set our aperture to around F16, to block out all the excess light. If, on the other hand, the subject was very far away from the light, then we’d set our aperture to around F4 in order to let much more light in. The inverse square law explains the dramatic drop-off in light over distance.

As you can see in the image above when the light moves further away from the subject the back round gets darker and darker and even some of the shadows on the subject’s face gets darker as well. When you see all three side by side, you really notice the difference of lighting. Make sure to remember, the flash is set to the same power level for all three shots. The only thing that’s changed is the distance to the subject (and the background).

This chart shows a visual representation of how and why the inverse square law works. And, remember, the numbers aren’t important. Just how the distance affects the light.