What is the inverse-square law in photography lighting?

Asked 8/2/2010

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2 answers

Photographers often mention the inverse-square law when positioning flash or continuous lights. What does the law mean, and how does changing the distance between a light source and a subject affect exposure in practical photography?

lighting photography-basics flash studio-lighting exposure

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

16y ago

2 Answers

The law states:

If you double your distance from a light source, the amount of light reaching you drops to a quarter of what it was.

More generally:

If you multiply your distance from a light source by X, the amount of light reaching you will drop by a factor of X^2 (X squared)

As is often the case, Wikipedia explains this very nicely (with a nice graphic, too).

Use for photographers

This means that you don't have to move very much relative to your light source in order to see a big change in the amount of light.

So, if you are lighting a subject with a strobe, you only have to move the strobe a little closer or further away to achieve a big difference in the amount of light reaching the subject.

Originally by user191. Source · Licensed CC BY-SA 4.0

user191

16y ago

AI Answer

Generated from our catalog & community — verify before relying on it.

The inverse-square law says light intensity falls with the square of the distance from the source. In practice, if you move a light to 2× the distance, the subject receives 1/4 as much light; at 3× the distance, it receives 1/9 as much light.

For photography, this means small distance changes can make a big exposure difference when the light is close. For example, moving a flash from 1 m to 2 m reduces the light by two stops. Moving it from 1 m to 3 m reduces it to about 1/9, a little over three stops.

Why it happens: light spreads out in all directions, covering a larger area as distance increases.

How to use it:

Move a light closer for much brighter illumination.
Move it farther away for less illumination.
When a light is close, exposure changes rapidly with distance.
When a light is farther away, the same movement causes a smaller relative change.

This is why adjusting light-to-subject distance is one of the fastest ways to control brightness in studio or flash photography.