Learning how to shape light is one of the best skills that you can develop as a cinematographer. Find out how with the Inverse Square Law.
Light is the most essential of a cinematographer’s tools. By diving into the physics of how exactly light moves, you can improve how you work with this fundamental filmmaking element. The Inverse-Square Law tells us, as filmmakers, what we can expect to happen to our subjects as they move away from our lighting setups — and how to control this process.
Distance
First, let’s look at an on-set example of how the inverse square law will save you time and make you look like a lighting wizard. Let’s say your key light is ten feet away from your subject. You’re getting a reading of 1.4 on your light meter, which is too dark for your lenses and would force you to up your ISO to a less-than-desirable degree to get the exposure. If a 2.8 is the exposure you’re going for, you can simply reduce the distance from your light to your subject by half.
The inverse-square law states that doubling the distance increases (or decreases) the exposure by two stops — or 75 percent. By moving the light from ten feet away to five, you get your perfect 2.8 exposure. By using the inverse square law, you can quickly arrive at this solution without too much headache.
Throw
One of the most practical applications of the inverse square law is in interviews. Let’s now say that you’re in a two-person interview setup. You set your lights and quickly notice that one of your subjects isn’t nearly as well-lit as the other. They’re falling into the shadow of your other subject and look noticeably underexposed. The inverse square law states that every time you double your distance from the light to your subject, you lose 75 percent of the light. Losing 75 percent of your light from three feet to six feet is a very drastic change. However, losing 75 percent of your light from ten feet to twenty feet is a much smoother and gradual progression.
The reason why you don’t like your lighting is most likely because your light is too close to the two subjects. By simply moving your light back, or doubling its original distance from the subjects, you drastically decrease the amount of contrast between your subjects, and you more fully light their faces, creating a much more pleasing image. Simply put, light sources that are further away produce a more even effect with less contrast.
Studying light, its quality, and how it interacts with its environment can make you a better and more knowledgeable cinematographer. Start with the inverse square law and see where the knowledge takes you.