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.
- Your light loses power as you increase distance from the light to the subject.
- 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.
Assignment entries for this chapter
Q&A Discussions
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Peter SalasI understand some what, but I still don’t get the inverse square law. I’m confused cause lets say my main light is 6 feet away from my subject. 6×6=36. I get that, but what do I do with that information to help me get a proper exposure. Please explain I really want to learn this, but I suck at math and this just has me thinking that maybe I should just stick to ETTL.
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Louis DiChello
I don’t know if you actually got an answer to this because it is old…I just joined. The key is in the graph…look at the slope of the line from 1 meter to 2 meters…it is at a very sharp angle meaning that the light falls off really fast initially…if you started at 2 meters and then moved to 3 meters, notice that the slope of the line gets more horizontal…meaning the light isn’t dropping as rapidly. Move the light further away, and you begin to approach a horizontal line showing that there is very little fall off at that distance. This is the desired effect with a group because you want as little fall off across the scene as possible when shooting a group. I wouldn’t worry too much about the math, just remember this…light in close falls off fast. Test this with a simple window light portait…put the subject right at the edge of the window and take a photo. Now move them further back away from the window and take another photo. You should see that the face transitions to shadow much faster when the subject is closer to the window. You can use this same effect to change the look of an individual portrait as well as making use when taking group shots. Also, TTL won’t help you in this area…TTL will set the camera exposure for you, but it cannot compensate for the inverse square law…the flash does not know what the size of the subject is that you are shooting, or the effect you are going for in the image…it looks at a scene through the lens and determines the exposure by the tonality in the scene. Even in TTL, you have to move the flash further from the subject to change how the inverse square law effects the image you aim to create. Let me know if that is as clear as mud, or if it helps. :-)
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I understand some what, but I still don’t get the inverse square law. I’m confused cause lets say my main light is 6 feet away from my subject. 6×6=36. I get that, but what do I do with that information to help me get a proper exposure. Please explain I really want to learn this, but I suck at math and this just has me thinking that maybe I should just stick to ETTL.
I don’t know if you actually got an answer to this because it is old…I just joined. The key is in the graph…look at the slope of the line from 1 meter to 2 meters…it is at a very sharp angle meaning that the light falls off really fast initially…if you started at 2 meters and then moved to 3 meters, notice that the slope of the line gets more horizontal…meaning the light isn’t dropping as rapidly. Move the light further away, and you begin to approach a horizontal line showing that there is very little fall off at that distance. This is the desired effect with a group because you want as little fall off across the scene as possible when shooting a group. I wouldn’t worry too much about the math, just remember this…light in close falls off fast. Test this with a simple window light portait…put the subject right at the edge of the window and take a photo. Now move them further back away from the window and take another photo. You should see that the face transitions to shadow much faster when the subject is closer to the window. You can use this same effect to change the look of an individual portrait as well as making use when taking group shots. Also, TTL won’t help you in this area…TTL will set the camera exposure for you, but it cannot compensate for the inverse square law…the flash does not know what the size of the subject is that you are shooting, or the effect you are going for in the image…it looks at a scene through the lens and determines the exposure by the tonality in the scene. Even in TTL, you have to move the flash further from the subject to change how the inverse square law effects the image you aim to create. Let me know if that is as clear as mud, or if it helps. :-)