Description: The inverse square law is a law of physics that explains a light source’s intensity in relation to the distance from a subject. In simple terms, it’s the explanation of the reason why light is brighter when it is closer to a subject and why that brightness diminishes as your move the light further from the subject. It provides mathematical information to help a photographer determine the amount of light that will be lost or gained by moving a light a particular distance.
Inverse Square Law
Inverse Square Law Extended Explanation
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.
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.
Inverse Square Law In Practice | Transcription
Below is a full transcription of the video above
This is my favorite slide. Inverse Square Pye-man, kind of like Pac-man, but I am Pye. You get it? Okay, you will get it in just a second. This lovely real world demonstration, or really my real world demonstration gives you an example of Inverse Square Law in practice. This is not really good practice, but it does show you an example. We have the light placed over here on the left side. These are all different frames, and basically here we are one foot, two feet, three feet, four feet, five feet, six feet away from the light.
Let’s say that at one foot we are getting a hundred percent of the desired light that we want. When we step away to two feet, you are going to notice that I am far darker than this shot right here. This shot is virtually on the edge of being blown out. This shot is one quarter of the intensity of this shot. As we step away, you can see me getting darker, and darker.
The interesting to note is that the difference between one foot, and let’s say three feet, is extremely dramatic in brightness. That is a huge difference there, but the difference between five feet, and even seven feet is a very small difference. It’s much smaller than that initial difference. What we are essentially learning is that the further we get from the light source, the light is now falling off at a much slower pace than up close to that light source.
We have seen it in Pye’s world. Let’s see another real world demonstration featuring creepy Joseph. What have we done here? Well, Inverse Square Line, and practice, and granted he would not take a crappy photo like this in practice. This is just because I want to show you all a creepy, crappy photo, but in this shot right here, what we have done is we basically placed the flash right to the left side of Olivia’s face. Joe’s distance is roughly one foot from Olivia. Olivia’s distance is roughly one foot from the flash, and this makes Joe’s distance from the flash double of what Olivia’s is.
If you look at Olivia, she is nice bright, and properly exposed, and if you look at Joe, he is about three, four stops under exposed. Especially when you start getting to the other side of his face, he gets even darker. Now Joe happened to creep into her a little bit to close on this one, but this is where the flash is basically four feet from the model. Now what we have done here is, all we are doing is adjusting up the flash power.
Every time we take a step away, we have to adjust the flash power up to compensate, because now the distance of the flash to the subjects is much further. Look, in this one Joe was basically double the distance of Olivia, from that flash. Where as on this side, Joe is now only one foot further than Olivia. Olivia is four feet from the flash, and Joe is five feet. You will notice that the fall off is much less dramatic. Joe is actually much more similar in exposure to Olivia.
Here we move the flash eight feet away. We powered the flash up higher a little bit more, and again Joe is a creepy six inches away from Olivia’s face, and she is enjoying it. She is loving this whole thing, by the way, but here Joe is roughly nine feet from the flash, and Olivia is roughly eight feet. You can see the exposures equal out. We are having to go more flash power, but the further we get from the light source, basically the further the group is, then the more similar the relative distance is. Now Olivia is only eight feet, Joe is nine feet, and we get roughly the same light.
When we are landing a large group, let’s say the group is ten feet wide. I might need the flash maybe twenty, or thirty feet away. Depending again on the angle of that light from the group, to be able to get everybody roughly the same brightness, and there is other techniques to this too that we will cover in Lightning 201, and 301 where we take the camera, I want to take the flash off the camera. Now, before we conclude this crazy crap test, a lot has been made way to complex, the Inverse Square Law. Let me give you four simple take-aways.
Number one. The Inverse Square Law still applies to individuals. Meaning if I am shooting a lovely model, whether it be a guy, or a girl. Let’s assume it is Olivia in this instance. In this shot I have placed that light so close to Olivia’s head, that if I am shooting Olivia full length, her head is going to be so much brighter than the rest of her body, and you can actually see that.
The flash is right here, and you can even see by the time it reaches her chest, I am only getting twenty, thirty percent of the amount of light, as I was on her face. If I place a light modifier super close to somebody, it is going to still have the same effect, even if I am just shooting them individually versus as a group. Remember, if I want equal light across the entire part of the body, then that light needs to be roughly equal distance from the entire body, from head to toe.
Number two. The more people you have in a shot, well in general it means the more distance you want to have. The larger the group, simply move the light away a bit further to get even light on everybody, and that means that you are going to need to power up the light. Remember you are losing more light than you think you are. Every time you double the distance, you quarter the flash power that you have remaining. Doubling the distance means twenty five percent the power.
Number three. The basic rule of thumb. When you are trying to light a group of individuals, let’s say a group of ten people, is as each person in this ten person group, are they each roughly the same distance from my light source. My light source is fifteen feet, twenty feet away, then a difference of one, or two feet between that relative distance is not really going to make to big of a difference, but if we are talking about twenty feet from the light on one side versus ten feet from the light on the other side, yes you are going to get twenty five percent of your light on that side, and you are going to get hundred percent of your light on the other side. Just remember, is everybody roughly the same distance from the light source?
Number four is the more dramatic, the angle of the flash, the more distance that you need to have the same relative distance in everybody in the group, and here is what that means. All this means is if my flash is directly on top of my camera, which again this really depends on what tools you have with you, and what you have, but let’s just say I am using direct flash on a group of fifteen people. From standing ten feet away, then my distance to all fifteen people is roughly the same.
It might differ by six inches to a foot, or so, as that direct flash is on my camera, and it goes out to each person in the group. Very little difference, I will get roughly the same amount of brightness on every person from that angle, but as soon as I take that flash off the camera, and let’s say I don’t even need to remove the flash from the camera. I can be bouncing from a reflector, or whatever it is. As soon as I move the primary light source away from the camera, it is becoming more dramatic. We are getting more shadows.
Now that light is going to be favoring one side of the group far more dramatically than the other side. As soon as that light is at an angle, the group that is on this side is now going to be probably far closer, maybe ten feet from the flash. Where as the group on this side is going to be twenty feet from the flash. The more dramatic the angle of the flash to the group, the more distance you need. The greater the distance, you have got to pull back even further, to get an equal light from right to left.
That is why, when doing a group shot, a lot of times they will place lights on both sides. The will cross light it so that everybody has equal light, or they simply shoot group shots with an individual light in each portion of the group, and that is something we might do in Lightning 201, and Lightning 301. We will show you advanced techniques for shooting large groups with off camera flash.
At this point I know all of you have absolutely mastered the Inverse Square Law, and you understand. You can explain it to anybody who asks, and it will be like, “”Look dude, this was totally so much more simple than everybody else explains it out to be, here it is.” That is what I want you guys to do. I want you to say it with that same exact tone, and accent, and everything like that too.
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