One of the more common mistakes many beginning and intermediate videographers make is confusing our tools for our medium. Too many shooters focus on camera and lens, but that’s like a sculptor obsessing over what chisel to use and ignoring the giant block of marble in front of him. Our medium isn’t our hardware. Our medium is light. Previously, we talked about quality of light. This time, let’s talk about luminance and the inverse-square law.
What is luminance?
First things first: even though some people will use them interchangeably, luminance is not the same thing as brightness. Brightness is a subjective judgment as to how bright a picture is. Luminance is an objective measure of how much light an object is giving off at a certain distance. It’s like the difference between wetness and volume of water: You can say “my lawn is wetter” or “my lawn isn’t wet enough”, but you would never say “my lawn is exactly 5 wets today”. That’s brightness. On the other hand, you can say, “I just watered my lawn with exactly 2 fl oz of water per square inch.” That’s luminance.
Brightness is a subjective judgement. Luminance is an objective measure.
Specifically, luminance is a measure of how much light is coming off of a particular object and hitting your eye — or camera lens. The current unit of luminance is the cd/m2, or candela per meter squared, though in old textbooks you’ll find it called “nit”, “foot-lambert” or “stilb”. Yes, “stilb.” Thankfully, that one never really caught on.
“But isn’t luminance measured in lumens, lux or foot-candles,” you might ask? Well, no, not exactly. Lumens are a measure of luminous flux, which is how much total light something gives off. A light bulb may have a high lumen rating but if most of the light is pointing in the wrong direction, who cares? Lux and foot candles are a measure of illuminance, which is how much light hits an object, not how much light makes it from that object into your camera.
Let’s go back to the water analogy again. Let’s say you have a drinking fountain in your house and you are trying to use it. The lumens would be like the total water pressure in your house. It’s good to have water pressure, but it doesn’t matter if all the water is going to the laundry machine. Illuminance would be how much water is coming out of the fountain — but who cares how much water the fountain is spraying if it bounces off your chin instead of hitting your mouth? The water that comes from that fountain and goes in your mouth — in other words, the light that comes off of an object and makes it into your lens — is luminance.
The Inverse-Square Law
Now that we know what luminance is, let’s talk about how to manipulate it. To do that, you need to understand the inverse-square law, which states that luminance decreases inversely to the square of distance. In formula terms, luminance = 1/d^2. Now, what the heck does that mean in practical terms?
Let’s say you have a light at full power, one meter away from your subject. If you move the light to two meters, doubling the distance, you might think that you would see half as much light on your subject. However, because of the inverse square law, you would actually get (½)^2, or ¼ the amount of light. If you triple the distance to three meters, you won’t get ⅓ the amount of light. You’ll get (⅓)^2, or 1/9.
This means that if you start off with a light close to a subject and move it away just a little, you will get a dramatic drop in luminance. Going from one meter to two meters means you lose 75 percent of your light in one meter. That is a huge shift!
But what if you start your light at full power at seven meters, instead of one? You’re getting 1/49th of your maximum light at seven meters. Move your light from seven to eight meters and you’re getting 1/64th of your maximum light. Well, going from 1/49 to 1/64 is a relative reduction of only 25 percent of your light, even though you moved the same amount going from seven to eight meters as going from one to two meters. In other words, the further your light source is from your subject, the smaller the change in luminance when that distance changes.
Practically speaking, what does this mean? If your subject is going to be moving around relative to your key light, you want to position your light as far away from your subject as is feasible, so that the luminance differential — the amount the light increases or decreases — is as low as possible unless you’re going for a big, dramatic lighting change. This also applies when shooting groups of people. Set up a light too close, and while the person closest to your key is well lit, the person furthest away might be underexposed. Sometimes you’re shooting in close quarters and you can’t get the light back far enough to avoid a dramatic drop-off. If that happens, you’ll need to get creative. Re-block the scene, use reflectors to bounce light around or maybe add a bunch of fills to your key.
This also applies while using the same source to light your background as you are your subject. If you want your subject to pop from your background, move the light closer and your subject a few steps out from the background. If you want them to be close in luminance, move your light out and have your subject step a little closer to the wall.
Of course, don’t forget what else changes if you move your light closer or farther from your subject. And it isn’t just the quality of light — consider reflections in windows and eyeglasses as well.
All of this sounds complicated on paper but once you start using the inverse square law in real life, it becomes intuitive pretty fast. Here’s your homework: Put a subject a half meter in front of a wall in a big, dark room. Set up a light at ¼ power, placed at whatever distance you need in order to correctly expose the subject. Start shooting. Now double the distance from the light to your subject, but don’t turn up the power on the light. How does the subject look? Now double the power on your light, to ½. How does the subject look? Now max out your power. How does the subject look now? How does the background look? Pay attention to both shadows and reflections.
Next, go back to ¼ power and do everything again, but this time start with the subject at two meters in front of the wall. How does your subject look relative to the background? Compare your first sequence and your second sequence. Let us know on our forums how it turns out.
Mike VanHelder is a working writer, photographer and video producer in Philadelphia, PA. His mother always told him that he was very bright.