I lost a one-dollar bet yesterday when I challenged a fellow-photographer’s understanding of the inverse square law. Turns out I was the one who had the math wrong. I don’t know where in my years of understanding this rule I got off track, but I sure was wrong yesterday. Hopefully it was just a momentary glitch. Anyway, I figured I’d review it one more time here in case anybody else has their math mixed up like me.
The inverse square law, as it applies to lighting, means that with every doubling or halving of the distance of a subject from the light source, the intensity of light doesn’t change strength equally. It’s not just half or double, it’s in fact four times stronger (when moving closer to the light) and a quarter the strength (when moving away from the light). In real world terms that means that when you have a light source four feet from a subject, a background (or secondary subject) eight feet from the light source will be one quarter as bright—or two full stops darker. A subject exposed at f/8 in that scenario would see the background register f/4 on a light meter.
In practice, this principle is used to make backgrounds lighter or darker, to cast even lighting over a wide area, and to frustrate photographers like me and win money in ridiculous bets about who knows more about photography.
Read more about the inverse square law and see great examples of it in practice on Zack Arias’ blog at www.zarias.com/white-seamless-tutorial-part-3-from-white-to-black