Now, don’t freak out! For some of you the words “Inverse Square Law” might bring back painful memories of that high school math class you thought was finally behind you, but we promise that it’s not that complicated. We will explain this concept using layman’s terms or as I like call them; “Pyeman’s terms”.

The Inverse Square Law can be simply understood if you keep these two things in mind.

1. Your light loses power as you increase distance from the light to the subject.
2. You will lose this light at a faster rate than you think.

For example, if you set up your light 1 meter away from your subject and you are getting 100% power flash hitting your subject. You move your light back 1 meter and now you’re 2 meters away. Does that mean you lost half your light, about 50%? It seems to be logical but it’s not the case. You actually lose 75% of your light. We can say this a different way. You only have 25% of your light intensity hitting your subject or ¼ of your light. Below, the chart is enlarged to help visualize this.

Before we move on, let’s quickly take a look at the actual formula.

Intensity=1/Distance²

Why is it called the inverse square law? Let’s break it down further step by step:

Simply put, inverse means the opposite of itself. You flip it. For example, 2 would be ½ and 4 would be ¼.

Simply put, square means a number multiplied by itself. For example, 2×2 or 10×10.

When we combine them, we get inverse square.

2 inverse is ½

½ squared is ½ X ½    

And what is ½ X ½? It is ¼.

So 2 inverse squared is ¼. Another way to say this in terms of photography is that at 2 meters away you get ¼ the power. This is the inverse square law. That is how you figure it out. If you know the distance you are going to be from your subject, all you have to do is plug that number into the formula to find out how much light you will lose.

When looking at our chart you also notice something very interesting. From 1 meter to 6 meters, the percentage of light loss is dramatic. But from 6 meters to 7 meters is not as dramatic, it’s only .6%. Also from 7 meters to 8 meters, it’s only about .5%.

Knowing these basics, the inverse square law is used most when shooting large group pictures. It has a practical sense. If you place your light front and center of your group about 3 meters away, based on our chart and the formula you might think that the group will be getting 11.11% of your light and you will adjust accordingly (increasing the power). But only the individual at the center will be receiving 11.11%.

The individuals at the far corners of the group are going to be farther away than the one at the center. They might be 6 meters away from the light source thus only giving them 2.78% of the power. This creates a very unflattering and uneven light for group portraits. Because we know that light intensity has a less dramatic falloff at a greater distance, we have to pull the light source even further away to have even lighting among all the individuals.

Even Lighting On Family

The inverse square law is not as complicated as it sounds and knowing these fundamentals will benefit you greatly.