The “inverse square law” regarding light dropoff says that light drops off with the square of the distance. I.e. an object 4 times farther away gets 16 times less light, and so on.
This law needs to be part of your DNA!
Why? Because it explains those dark flash backgrounds. And because it helps, too. Take this shot of my model Kim in a grungy garage, using an off-camera TTL speedlight through an umbrella on our right:
Fine. But what if we wanted a darker background? Remember Willems’s Dictum: “bright pixels are sharp pixels”.
The solution is simple: move the umbrella closer to her. Then the background is farther away in relative terms, so it gets darker with the square of that ratio. So now we get:
And if we move ourselves to get the umbrella out of the picture, here is what we end up with:
Simple solution to a vexing problem if you like dark backgrounds!