The Inverse Square Law
Now it's time to talk about one of my favorite things in photography. It's called the inverse square law. And once you've mastered this, it will really change the way you take photos and understand light. And so first, let me just read to you the Wikipedia definition of the inverse square law. I've got a slide for this and it says that in physics an inverse square law is any physical law stating that it is a specified physical quantity or strength and it's inversely proportional to the square of the distance from the source of that physical quantity. Okay, so, uh, that doesn't help us at all. So, a different mark Wallace Layman's term version of this says that the inverse square law when it comes to light is that the strength of the light is inversely proportional to the distance of the light to the subject. So let me show you using graphs and charts what that means. So, I have this fancy graph. So let's say over here on the left hand side, we have a light. And on the bottom of this gr...
aph we have the distance. So one ft 2 ft 345. All the way up to ft. Now we can look at these numbers. So the inverse of two. So two squared is four and the inverse of four is 16. The inverse of six is 36 et cetera down the line. So two times two is four. The inverse of that is one quarter four times 4 16. The inverse of that is 16. You can see how that plays out down the line. So what that means is that at one ft our light is at full power at two ft twice the distance, the output of our light drops drastically to one quarter power at four ft, it's 1/16 of full power. And you can see it goes all the way down to a 36 a 64th. All the way down to 1 110% 91% of total power. It's nuts. And so to understand this a little bit better. I've converted these numbers into percentages that looks like this. So from one ft we have 100% full class and we go to two ft our light has dropped to 25% of full power and four ft notice something is happening here. So from four ft to 10 ft, we only go from 6% full power to 1% full power. So there's only a 5% difference between four ft and 10 ft. Or as the difference in power from one ft to two ft Is 75%. It's crazy. And so if you had a little line to sort of graph this out, it would look something like this. I'm not a very good artist and my lines are not very good. In fact, it's horrible. It's miserable. But it gives you an idea. We have a cliff here. So the light falls off dramatically close to the light. And then it evens out. And so that has really large implications for our photography. Think back when we were shooting with Quinn and the light was really close to Quinn, The background was almost completely dark because that cliff, so the light was exposed correctly on Quinn and then right after that it dropped off by 75% and then the background didn't have enough light to show up in the image, it was really, really dark, but when I moved the light really far away to show that hard light, uh kind of stuff with the effective size of the light, remember the background was white? Well, the reason for that is we met metered for that tail, that really, uh low tail at the end of that light. And so the light on Quinn was essentially the same as the light on the background. And so Quinn and the background were both exposed correctly, which means that the background was white. We're gonna do this, but that's sort of how that works. Now, there is a caveat and that is that this law. This law applies to all kinds of things by the way, not just light plays to sound and many different things, but it uh, it assumes that you have a point source of light around dot of light like the sun or a light bulb, something like that. And so when we look at that in physics, we get this kind of thing. And this sort of helps us understand what this means. So with the point source of light around sphere As the light travels away from that source of light, it travels, the photons are traveling in one direction. And so what's happening is as they're traveling out the traveling in a sphere, so 360° and the farther they go from that point source of light, if you look at any square in the radius, the farther away you get from that light, fewer and fewer photons are hitting that square. And so the light is diminishing according to the inverse square law. So that's pretty cool. But um we're not always shooting with point sources of light, in fact almost never are we shooting with point sources of light? We're shooting with diffused light. And so that light is scattering all over the place, It's bouncing off walls and ceilings and floors and clothing and reflectors. And so you might find that you get results that are inconsistent when it comes to moving your light close and far away from a subject, that might be because the light is bouncing around and doing some things that is affecting how the inverse square law works because it's assuming it's from a point source, it's only coming from one place and moving outwards instead of coming from a bunch of different places, bouncing off all kinds of things reflectors and soft boxes and stuff. And so just be aware of that, that the math is not gonna work out exactly like I just showed you because of the way that the laws of physics work. Okay, so we are about to do a demonstration. I'm gonna roll this off the stage here now, so this is sort of how this would work. So Quinn is going to come out here and Quinn, I will have you stand right here, Quinn is going to stand right here and remember we have to see this by the way, we're gonna go really wide, we're gonna widen out our shot and then I'm gonna move this over so that we can actually see what's happening. So matt is going to take his camera and he's gonna go actually, let me do a demo first, matt, let me show you this, I'm going out of out of sequence. So if Quinn is here, um so Quinn, let's have you face this light and we're illuminating her from this way. Remember we have the inverse square law, so the light's coming out right here and I would meet her the light on Quinn right here and get a value and say this is how much light is here on Quinn for a correct exposure, But then the light, Whoa, it falls off down here, we have that long trail and by the time it hits this background, remember we've lost like 75% of the light. So Quinn is going to be exposed correctly but all the way back here. Darkness unless it's a Quinn, I'm gonna have you're gonna put you back way back. Just trust me, it's like a trust fall, come on back. All right. So let's say Quinn is here now, what has happened is this light does its dramatic fall off? Just like it did before and it evens out. So now if I meet her Quinn here, I'm going to get a certain value. But guess what? Because almost all the light has already fallen off the amount of light that's here, it's pretty much the same as the amount of light that's over here. And so if Quinn is illuminated correctly here, well, the background is probably illuminated correctly as well. So that's why this background is going to look white when she's far away from light and black when she's close because we've metered differently. Okay, watch this. That's gonna go removing him over and we are going to prove this. Who? That was really fun. Okay, so what I'm gonna do is I'm going to turn on my light and it doesn't really matter which light modifier you use for this. I'm using a smaller light modifier because of what I explained earlier with the point source of light versus a diffused light, you're gonna see light fall off more dramatic with hard light sources because they are more closely aligned to point sources of light, then you will with giant soft boxes etcetera, you still get the same laws of physics are just more pronounced with smaller lights. So we have this road box here, it's about a foot, something like that. It's not very large. So what I'm gonna do, it's turned on, let me grab my camera here and yeah, it is on. Okay. So what I'm gonna do here is I have my metering mode and T T. L metering. And so this camera is going to look at Quinn's face and try to make sure her face is exposed correctly because it's using face priority metering. So look straight at me. I'll take a photo and believe it deep. I think my camera did not take a photo. How weird is that? Maybe it wasn't on. All right, one more time. Here we go. Okay, So, Oh yeah, I did. I have to learn how to use my camera. That's what the deal is. So when we look at this, it looks underexposed because of the TT L metering. Try one more time. See if we can get our exposure to be correct. All right, so T. T L metering is sort of messing us up. That's okay. The important thing is look at the light on Quinn and the light on the background. They are pretty consistent there. And so that works pretty well. Um so to make this work a little bit better because tcl metering is driving me nuts. I'm gonna shift everything back into manual mode because this will not do. And so okay, so I'm in manual mode, I'm gonna go over here and manually meter this light because I really want you to see this. Um it needs to be correct. So I'm gonna meet her. This, That meter's at 6.3. If I'm entered the background, It meters right at 45, so there's a little bit of a difference. So let's try this out. So here we go. 6.3 is what I said, Yep. Okay, so Quinn looked right at me. Excellent. Perfect. Okay, so now this is going to pop in. Huh? Thank you. Now we have a proper exposure. So look at that question in the background are very, very similar and exposure because of that drop off. So Quinn, what I'd like you to do is Quinn's gonna walk forward. I want you to walk to about right here, So you're taking that's about eight steps, something like that. Just about six ft. Um so just go to the left yet, that's about right, just like that. Okay, so now we're about three ft from the light, so it's not very far away. Let me meet her this And that's really bright. That was F 16. So we'll do that again. Okay, F eight or F eight now, So to F eight, we're going to take a shot of Quinn. Perfect. Meeting bam, just like that. This is going to pop in and look at the background, look how much darker the background got. So if we look these two side by side, it's a dramatic difference and the reason for that is the inverse square law. And so the crazy thing about this is a lot of times if you're shooting and you have a person that's far from the background and it's dark. A lot of people will try to move the light closer to brighten that background, but watch what happens. I just move this a little bit closer. So now we're not even a foot away from Quinn. So I'm gonna meet this one more time and that's a little bit bright and take this down just a bit, lets me do that at metered at 12.7. So that's What is that? 13? Yeah. F 13. Okay, So I'm gonna do this at F bam. Okay, now look at that. Holy Smokes. What a difference. What a difference. Look at these three side by side. The only thing that changed is the distance between the light and Quinn. Um And so Quinn was closer and closer and closer to the light. The background got darker and darker and darker. So that last shot, the background is black. Black. And the front shot, the first shot, the background is really light gray, almost white, not quite white. And so we have different shades of gray from one background. So a lot of times I'm making videos or whatever. Uh specifically on youtube. I'll shoot something like this and people will write to me or comment and say clearly you're faking your shots because that's a white background and in the picture it's black, but I'm not. This is what happens when you have the inverse square law. So let me just once again show you the distances. So this is the background 12345, that's about 15 ft 10 ft, something like that. And the distance from Quinn to the light is two finger widths, so that's not even a foot, something like that. And it will change so quick. Just take one big step back. Okay, And then we we just had one big step back. Now let me meet her this again. 6 3. So with that meters. All right, 6.3. Here we go. Let me take another shot. Here, click. Let's look and see what that does for the background. Now, it's like gray, it's a light gray. So we went from black to a light gray and we just moved a little bit. It's crazy. Now I know what you're thinking, you're thinking maybe uh mark what the heck you're shooting with an aperture value? That's changing. So maybe that change in aperture value is what's changing that background. So let's do this. Let's try doing the same thing at different distances and I will try my best to always shoot, let's say f 63, I think we'll have enough power to shoot at 63 So I'm gonna meet her until we have a 6.3 aperture value every single time. And let's see the differences because I can tell you it's going to work so Quinn come really close again. We're about a finger away about a hand away. So I'm gonna meet her this to the light that's 14. So I need to go way down See if we can get this to 6. seven. Let's see if we can go, How low can we go? Oh that's 5.6 I'm gonna make sure it's perfect. 6.3 exactly 6.3 Okay, close shot. The closest shot 63. So close I can barely even get this in there, click OK 6.3 Super close here it is. Let me bring this up, you can even see over here That is 6.3 on the screen Right there. 6.3. Can I do it? Yeah 6.3 that's what I shot. Okay so look at that background, it's very very dark. All right now what we're gonna do is take two big steps back. There you go. Yeah right there that's good I'll meet her this, It's 1.8 we need a lot more light You want to get it to 6. four almost almost there 5 6. Oh so close the tension is killing me. The silence foreboding seven or too much if you played this fast, it would probably look funny like I'm going back and forth, 6.3 exactly, yeah. Okay, now just try it one more time. 6.3 click this is going to come up bam just as we said, it's a brighter background. All right, go tell you about half an arm's length from the background, yep, that's good. Let's see if we can get 6. seven Oh, so close, so close. All right, here we go. You know, I guess I could have you doing this. That was 6.3, Exactly. In the future. When have you do it? I don't know why I didn't think of that. All right, so Quinn knows that a meter. What am I doing? Perfect bam. Now let's look at the background white, How cool is that? The inverse square law is a mini wondrous thing. Look at what we were able to do with the background just by changing the distance of our light from our subject. Um the other thing that we can do with this, that is really cool is it can help us solve issues when it comes to shooting groups. The reason for that is so quick. I want you to come and stand right here, be right here if you're shooting groups and you have like a school session or something. So you have the front row of people, you have another row of people, you have another row of people, You have another row of people a lot of times. What will happen is the front row of people are gonna be overexposed in the background. People are gonna be underexposed and it's gonna be an issue when you're shooting with flash because the light is too close. So the solution of that, it's two things. So number one, let's use a larger source of light. We'll do that so that we can get softer light across a larger area. And because our light is so far away, the effective size is smaller when we go farther away. So we need a bigger source of light. So we're going to use something much larger. So this is the same shape as the other guy. It's just bigger. It's like twice his size, maybe three times the size. So the other thing that we can do is we can move the light far away to prove this. Gonna have matt come out here. He's going to be a volunteer. So matt, what I would like you to do is stand back here. Bye. The seamless. And I'm gonna move this light back back back. Okay, This guy is a 500 watt second line. I probably should have used 1000 1 2nd light to get a little more punch. It's at full power. Let's see what we can get. So I'm going to meet her. I'm gonna meet her over here on Quinn Bam. That's F nine when I meet her back here where Matt is Band at 6.3. So we still have a difference between the front and the back. But let's see if we can get away with it. Okay, So we're gonna shoot, I'm gonna shoot at F eight. I'm gonna split the difference just a bit. And here we go. Everybody is happy first day of school. Take a look at this. So matt is a little under exposed, but the distance between matt we're like six ft or about six ft. So matt take a half step forward or maybe one step forward a little bit more. There we go. About like that. So we're still Probably had two or 3 person group depth. Yeah, we probably have room for a couple more people there. And if I shoot this, you can see that now, it's pretty acceptable. So we could add maybe another light up high. We could do some other things here to bring out a little bit. But the key to this is to add some light at a distance. Use a big light modifiers so that when the light is dropping off, you get consistent exposure across a big space of maybe five or 10 ft. You might be asking yourself, but mark, I shoot outside groups all the time. I have none of this issue. Well, the son is 19 5 million miles away, whatever that is. So you get the lot of the inverse square lot comes into full effect because that is so far away that the exposure is pretty much consistent across the entire continent. If there weren't clouds, it's pretty cool. All right. Thank you very much, guys. So we have learned about the inverse square law, the angle of incidence, and, well, no, we haven't. That's something we need to learn about. Okay, we're gonna take a break, we're gonna come right back and we're gonna keep chugging along.