Shining a Light on the Inverse Square Law and How to Apply It When Lighting a Scene

This is one of the more important lighting concepts you'll have to learn as a filmmaker.

Even though we'd like it to be as simple as flipping on a switch, lighting cinematically requires a deep understanding of how light behaves in different situations with different subjects, surfaces, and materials. Though there are many concepts you'll have to learn to really grasp these qualities of light, perhaps one of the first and most helpful is the inverse square law, which helps you determine light fall off.

If you want a more advanced explanation and demonstration of the inverse square law, photographer Peter Hurley talks about it in this video tutorial from Fstoppers.

However, if what you need is a good primer to get your started, David Bergman of Adorama TV gives a great introduction in the video below.

The inverse square law formula for calculating fall off is: Intensity = 1/d².

If you're not a math whiz, the inverse square law probably looks like school to you. However, applying it doesn't have to be homework; you can apply it in very practical ways. Bergman says,

Here's the only thing you really need to know: the light falls off a lot faster if the light source is closer to your subject. This is really useful to know so you can control the ratio of light on your subject and the background.

Though the math can be tricky, understanding the basic idea behind the inverse square law will help you make more educated decisions when lighting a scene.     

Your Comment

5 Comments

It is simple:
Twice the distance = 1/4 of the light.

So, for little fall off the light source should be further away than the distance it has to travel in frame.

If the light is 1 meter outside the frame and it travels 1 meter in the frame the fall off is:
1/(1^2) - 1/(2^2) = 1 - 1/4 = 3/4 = 75%

If the light is 6 meters further away:
1/7^2) - 1(8^2) = 1/49 - 1/64 = 0,004... = 0.4% fall off.

The caveat is that the nature of the light changes as well: the further away, the smaller the light surface, the harder the light. Large diffusion screens with multiple lights are the solution for that.

May 9, 2017 at 5:29AM

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WalterBrokx
Director, DOP, Writer, Editor, Producer
9925

If the distance from the light to the subject doubles, you must quadruple the amount of light to achieve the same exposure.

May 9, 2017 at 7:16AM

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Richard Krall
richardkrall.com
2157

Really cool.
Thanks a lot.

I really need some help on mixing day light and warm light.
I have Fancier LED 1024AC.
When I mix cool light and warm light, something goes wrong in the color grading.
Please help me to find out what I am doing wrong.

May 10, 2017 at 1:45AM

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Sameir Ali
Director of Photography
1656

Don't mix warm and cool light. Unless you are going for an effect.

May 11, 2017 at 6:13AM

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Richard Krall
richardkrall.com
2157

What Richard says: don't mix unless you want the blue vs yellow in your image.
You need to gel your light to make it match daylight. The Downside is you'll lose light by doing that.

May 12, 2017 at 2:29AM

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WalterBrokx
Director, DOP, Writer, Editor, Producer
9925