How to Use the Inverse Square Law in Photography

Lighting  •  Photography Tips  •  Portrait Photography

If you need a one line explanation of the inverse square, here it is: “It’s basically light fall off.” That’s really all there is to know. But that’s probably not why you’re here. You’re here because you want to know how it impacts your photography. Peter Hurley explains:

For starters, this is the mathematical formula for the inverse square law:

Intensity of Light = 1 / Distance²

This is the only mathematical formula that you need to know if you’re a beginner photographer.

Very simply, light fall off—or drop in intensity of light—is huge over initial distances. For example, the change between one foot and two feet.

Light Fall Off

When you apply the formula, it becomes easier to calculate how much you need to pump up the light in order to get a decent exposure.

The formula is simple (contrary to popular belief). All you need to do is take the distance from the light to the subject and then inverse the square of it. So, if the distance is one foot, the inverse of one squared comes down to one. In other words, the light you get is 100 percent—no adjustment is necessary.

When the distance becomes two feet, you get inverse of two squared which is 1/4, which means you get only 25 percent of the light. When the distance is three feet, you get 1/9—only 11 percent of the light—and so on.

Notice a pattern in how much the light drops when you double the distance. When you move from one foot to two feet, the amount of light loss is 75 percent (100 percent to 25 percent). Then again when you move from two feet to four feet (doubling the distance), the amount of light lost is again 75 percent (25 percent to 6 percent). This goes on.

How does the inverse square law impact your photography? Let’s say that you want a contrasty image where the subject has a strong shadow of his nose running across his face. Place your subject as close to your light source as possible. And if you need your subject to be lit by a flat, uniform light, move him as far away from the light as possible.

Above is an example of a dramatic light fall off. The model’s face is well lit, but his ears are almost obscured.

Here’s the same shot with the light moved a couple of steps back:

You can also use this technique to make a background appear white or grey or black. For example, in this shot, the light is far away from the subject.

f/2.8, 1/60 of a second, ISO 100

Notice the background. It appears fairly white. Remember, light fall off is at its maximum when the light source is at its maximum distance.

f/5.6

With the light moved closer (above), notice how the background is noticeably gray. This is because of light fall off.

And in this final shot, the light is even closer. Notice how the background has turned completely dark.

f/11

The background can be turned completely black by moving the light even closer and or moving the subject farther away from the background.

So, that’s the inverse square law in a nutshell. Try applying it to your next photo shoot and let us know how it goes!