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What is the Inverse Square Law of Light?

The simplest explanation of the inverse square law of light, and how it is one of the most powerful tools in your arsenal.

In this video and article I’ll explain the inverse square law of light and how it is one of the most powerful tools in your arsenal.

First, watch the video:

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What is the Inverse Square Law of Light?

Simply put, light loses energy as it travels. The further away you go from a light source, the dimmer it gets.  This is why there’s an ‘Inverse’ in the name.

Now as to how it loses its energy. We intuitively expect light to drop off (“fall off” is the commonly used term) linearly. You move twice as far, it should fall off by twice as much, right? Nope.

Light falls off to square the distance. You move twice as far, it falls off by four times. You go 10 times away, it falls off by 100. That’s why there’s a ‘Square’ in the name.

Fall off is proportional to square of the distance between two measurement points.

You measure the light at one spot, and then another spot. The distance between the two points dictates the fall-off. Don’t worry, you don’t have to do any math.

It’s a law of physics – one that you can use to your advantage.

Why is it absolutely critical you know and use it?

Unfortunately, if you’re ever going to light a project you can’t escape the inverse square law. It’s a critical tool for the cinematographer and photographer.

As explained in the video, the closer the light source to the subject, the faster the fall off. The further it is away, the smaller the fall off. That’s why, even though the sun is millions of miles away, it lights the land evenly.

But stand next to a window and the shadow side of your face is much darker than the lit side.

When lighting locations, especially large ones, it’s important to keep the inverse square law in mind or you’ll just screw up. But once you get the hang of it, it’ll be second nature. Practice makes perfect.

Easy way to remember it

The easiest way to remember it is to forget ‘square’ and ‘double’ and what not. Cinematographers light in terms of stops. A change of one stop is double (or half) the light.

That means, every time you double the distance, you lose two stops.

Double the distance = Lose two stops

I hope this video and article has helped you understand the inverse square law. Let me know if you have any comments or questions below.

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2 replies on “What is the Inverse Square Law of Light?”

Pete Yodasays:

I am learning to be a Cinemetography it’s taking a minute to grasp the lighting concept.. but you video are so informative it’s forcing me to think outside box .. I just want to personally thank for your great videos . They have change me life ..

Sareeshsays:

You’re welcome!