Lighting a background with a single light sounds simple until you see the ugly gradient running from one side to the other. If you shoot portraits or products against seamless paper, getting that background clean and even can save time, keep your images consistent, and give you more control over how everything looks in camera.
Coming to you from David Bergman with Adorama, this clear video breaks down even lighting of backgrounds. Bergman starts by reminding you that every point on the background receives light based on two things: how far it is from the strobe and how strong the beam is in that direction. The distance side is the inverse square law, where doubling the distance cuts the light to a quarter and costs you two stops. On a long background lit from one side, the near edge is closer and brighter, and the far edge is farther and darker, so the distance alone gives you an uneven wash.
Bergman then shifts to the reflector itself and why its behavior matters so much. A standard metal reflector is brightest in the center and weaker toward the edges, which means you get a hot spot if you point it straight at the middle of the background. Instead of aiming the hot center at the side that is already closer, Collins suggested the opposite: point the hottest part of the beam toward the far edge that is losing light to distance. The near edge, which is closer, only sees the weaker outer part of the beam, so its “distance advantage” gets knocked down. When you balance those two forces just right, the reflector falloff and the distance falloff cancel each other, and the background starts to read as even from side to side.
The video also shows how this idea connects to what you probably think of as feathering. When you aim a soft box slightly past your background, you are choosing to work with the smoother, weaker edge of the beam rather than blasting the hot center straight at it. You see why backing the light up helps too, since greater distance reduces the difference in brightness from one side to the other before you even start worrying about beam shape. There is also a useful reminder that once you know how to make a background perfectly even, you can flip the script and deliberately create a gradient or hotspot instead of stumbling into one by accident. If you want to see exactly how the angles look and how small changes affect the exposure, you will get more out of watching Bergman do it live than from reading any description. Check out the video above for the full rundown from Bergman.
If you would like to continue learning about how to light a portrait, be sure to check out “Illuminating The Face: Lighting for Headshots and Portraits With Peter Hurley!”