Photo Breakdown #2
The Most Practical Explanation Of The Inverse Square Law You'll Ever Read
by Benjamin Weatherston - September 27, 2011
For a shoot in the October issue of The Ann Magazine I decided to shoot on the cyc wall and light for variety. My original idea, an outdoor shoot, drowned in 4 days of heavy rains. I was going with plan B and I knew I needed to be able to shoot any pose imaginable to give the designer enough for the cover and a few insets/spreads. Oh, and the whole shoot needed to be done in less than an hour so no relighting between shots.
There's a little trick I use for lighting the cyc in a pinch and it has to do with the inverse square law of light. I'll tell you what the trick is but first you have to listen to my smarts. The inverse square law is an obscure property of light that almost every photographer knows about but few can adequately explain, let alone use to their advantage. It states, “the intensity of light radiating from a point source is inversely proportional to the square of the distance from the source”. A simpler way to say that is, "illumination decreases by the square of the distance." The simplest way to say that is, "double the distance, quarter the light".
This is not to be confused with the Inverse Square Law of Photography which states "the more pictures you take, the less you show." Or the Inverse Square Law of Photographers which states, "For every two lost hours of sleep, drink four cups of coffee." Or the Inverse Square Law of Professional Photographers which states, "The more money you earn from shooting, the less pictures you'll take of your kids."
What happened to a practical explanation? Let me take us way down to super simple and then I'll prove I'm smart again. You are shooting a headshot with 2 lights (a main in front and a hair light behind) and then decide you want a 3/4 length to show off his fancy new belt buckle. You ask him to take 2 steps back, snap, and your picture looks awful. This is because he was 4 feet away from the light and now he's 8 feet away. So you think, "Man this stinks. I need to turn this light up!" while you tell the client, "You blinked, we need to take another one." So you discreetly boost the light from 1/8 to 1/4, snap, and your picture looks less awful but it's still not properly exposed. So you think, "I suck at taking pictures!" while you tell the client, "You blinked again, I'll count to three this time." Now you're laughing nervously, counting to three, madly spinning your EV+/- dial like a roulette wheel, praying for something close to a decent picture, planning all the post you'll need to do, and trying to remember if your camera is set to shoot RAW. Don't ever say a photographer can't multi-task!
In this case the subject doubled the distance but instead of getting half the light he got a fourth of the light. The light needed to be bumped up to 1/2 power not to mention the adjustment of the backlight.
So here's the smarts and they come in parts. Smart part 1: Light radiates out in all directions from a point source. We talk about light "falling off" as if it loses it's power or intensity but that's not really true. Each individual ray of light is still as strong, there are just fewer of them falling on the subject. The light isn't falling off, it's spreading out. Think about it, if the actual power of the light was cut by 75% in the 2 steps your subject moved back, there's no way we'd ever see the sun.
Instead of 4 "units" of light hitting his face, it's only getting 2. And this is only showing a vertical distribution (forehead to chin). The same thing happens horizontally (ear to ear). That's why doubling the distance quarters the light.
And now you're probably thinking, "Light sucks, I'm going into web design." Well wait, at least finish reading the blog.
Smart part 2: This isn't always a bad thing! Sometimes it can be used to your advantage. So let's break down those light rays into 2 types, steep rays and steady rays. Don't look for those terms anywhere, I made them up.
How can this be helpful? If you have big lights (or big things to bounce them off) and can move them far away from your subject you will start to see how cool those rock steady rays can be. They'll hit your subject, anyone standing next to them, and the background with the same intensity.
Let's say you're lighting a group portrait with a single off-camera flash. Your group is 10 across and 4 deep. The most effective use of that light is to move it away. Pump it up to full power and put as far back as possible. You will waste all that light that goes up, down, and off to the sides, but the light that goes straight out will work extra hard for you. The front row won't be overexposed and the back row won't be underexposed.
Here are two pictures I took at the Ann Arbor Shutterbug 1-Year Anniversary party we had at the studio. I was doing a demonstration to show how this works and took the same picture with two different types of light. Initially, we set up three lights pointed right at them about 12 feet away (in front and each side). Look at the shadows towards the middle of the group and also the light hitting the faces towards the edges. On the left side of the group you can clearly see side light coming in from the left and the right side of the group is hit from the right. If you look at the middle you can see the front row is getting more light than the back row. And quite obviously, the floor in front is much brighter that the back wall. This is not a horrible way to light a cyc wall but there's a better way (especially if you only have three lights) and here's the trick I used for the shoot with The Ann.
Turn the lights around so that they hit the wall. This does two very important things. First, it makes the light HUGE! The difference between a large and small light source cannot be understated. The shadows are now so soft they seem to completely disappear. Secondly, it maximizes every inch of distance between the light and the subject. Now instead of being 12 feet away each light is about 25 feet away. Since the wall is the light source, there is no possible way to move the lights any further. The distance is what makes the lighting more even across the group.
So with only three lights you can effectively light a very large area because you've made them big and far away. What I then like to do is use this as a foundation and add more light to the subject for whatever the purpose of the photo is. For the photo below I just added some sidelight to help define Jakob's chiseled features. (Note: I honestly tried to include a lighting diagram but the distances were so large that all the symbols became microscopic. You couldn't tell what anything was. And my efforts to make the lights/subjects bigger just ruined the scale. Sorry.)
"But what if I'm never going to shoot anything on a cyc wall or shoot large groups?" I knew you were thinking that. Effectively using the inverse square law can help with any type of picture in which you are responsible for the light. Even if the light is the sun, you can use distance and size to your advantage.
I know you've been told by plenty of people to get your lights in super close to increase the softness of the light. I'm not telling you that's bad because that might be what's best for a certain shot, maybe even the majority. (Hear that sound? That's a soapbox being dragged out.) But too often we know WHAT to do without knowing WHY or sometimes even HOW to do it. We have watched so many tutorials and read so many books that we've been told techniques, preached preferences, and given guidelines without much explanation. And, to be fair, sometimes we just totally ignore the explanation. It might be there, but we don't really care once we've gotten the tidbit of info we wanted. (What's the sound of someone being dragged off the soapbox by their own conscience?)
So, back to taking pictures. If you were shooting a couple and you wanted a nice sidelight I would go against the advice to move lights in close and actually use the inverse square law to your advantage. Here's a shot of Jen and I with a small softbox 2 feet away from my face standing in front of a white background. I am slightly overexposed while Jen is underexposed and the background is completely black. I am not casting a shadow on her and there are no reflectors for the sake of this demonstration. There are two factors at work in the exposure of this photo. One is the size and the other is the distance. The harsh shadows are caused by the size of the light. The difference in the amount of light is caused by the distance. Unfortunately, we usually only think of fixing this problem by increasing the size or rotating it around to the front (losing the sidelight) and not increasing the distance. Not to mention that there are probably some of you who just thought, "dodge and burn." Excuse me while I shed a tear for the future of photography. This might be nice for a super moody portrait of a man who oppressively keeps his wife in the shadows while he lives it up gallivanting around the world taking high fashion portraits of super models in exotic locations...but if you know Jen and I...yeah, no.
Here is what happens when we increase the size. This is the same light at the same distance but now inside of of the Chimera 7' strip. The shadows are softer and the white background is now a dark gray. But even with a fancy softbox, we are not solving the exposure problem. Jen is getting a lot less light and the background is too dark. Now I happen to like a dark gray background but what if this is someone's home or an executive's office?. They might like to see what's behind them.
Here is what happens when we increase the distance. This is the same light in the small softbox simply pushed back 10 feet. Now the lit side of my face and the lit side of Jen's face are the exact same exposure. Amazing! And what's up with all that fill? Well those steady rays are actually flying past us another 12 feet to the studio wall and bouncing back at us. Doesn't it seem silly to think that light actually diminishes in power that fast? Yeah, it's almost as silly as this weird grin on my face.
Here is what happens when we increase the size and distance. Now we get the best of both worlds, soft shadows and even exposure. With one light we have effectively lit both people and the background.
For a situation like this, increasing the size and distance is almost always preferable. But in a pinch, I would encourage you to increase the distance before the size. On an actual shoot it will almost always be more practical to back a light off and power it up rather than switch out a softbox (if you even have another).
So now that you're smarter than 85% of all photographers on the topic of the Inverse Square Law, I would encourage you to practice these principles. Knowledge is good, experience is good, knowledge and experience is best. But what I've seen, in myself and many others, is that knowledge without experience eventually fades. Maybe it has it's own Inverse Square Law?
GI Joe said that "Knowing is half the battle." And while I felt very motivated to steer clear of fallen power lines immediately after the show, they obviously seemed more appealing the next day. So maybe the Inverse Square Law of GI Joe states that "knowing decreases by the square of the time." Or more simply: Tomorrow you won't remember this stuff, go out and practice!"