In fact, studio lighting—and lighting in general, for that matter—is all about ratios and relativity. The strength of the fill light matters in relation to the strength of the key. The brightness of the background matters in relation to the brightness of the key. It all comes back to the key light. How the background is illuminated relative to the key light determines what the background will look like in pictures. That’s why what you see isn’t necessarily what you’ll get. And why, in the end, “it won’t look like that.”
Take a look at the examples here. They all use the same seamless paper background—specifically Savage #53, Pecan. It’s a lovely light brown that’s great for portraits and with thoughtful lighting can become very light brown, very dark brown or anything in between.
Set up in the studio, I meter the key light to determine an appropriate exposure that makes the subject appear normally illuminated. In this case, it was ƒ/8. Based on that exposure for the key light, I then use another light to illuminate the background and meter it to determine its exposure. If the meter when placed on the center of the paper also reads ƒ/8, then you can be assured that the paper background will look like what you see normally. When the background light produces the same exposure as the key light, the background will look like it does naturally.
From here, you can choose to over- or under-expose the background to change its appearance, making it lighter or darker as you see fit. If you move the light farther from the background or dial down its output, you’ll underexpose it and make it darker. If the background light meters at ƒ/5.6, it will appear one stop darker than it does to the naked eye, based on the fact that you’re shooting at ƒ/8 for the subject.
Dial up the output on the background light, or move it closer to the background, and it will make the background appear lighter thanks to overexposure. If the meter now reads ƒ/11 at the background, it will appear one stop overexposed when shooting at ƒ/8.
Of course, moving the light closer to the background has another impact too. Namely, it can create a hot spot in the center with pronounced falloff at the edges. This is often desirable in portraiture, and in fact, many photographers use a grid-spot or snoot to focus the background light into a circle of light that appears brightest behind the subject and falls off to darkness above their head and toward the edges of the frame. This natural vignette is another great way to make even a perfectly flat paper background appear nuanced and enhance the illusion of depth on a two-dimensional plane.
Remember that this works with all kinds of lights: off-camera flashes, big studio strobes, tungsten hot lights, LEDs, fluorescents or even daylight (if you could figure out how to modify it effectively). With the flashes and studio strobes, you won’t see with your naked eye how the ratios will work in-camera, though you’ll get a much better idea of that with continuous lights. Still, the principles remain the same.
The next time you set up a seamless paper background—or light any background in a photograph—remember that you can dramatically alter its appearance by increasing or decreasing the amount of light on the background relative to the key light. This way, a handful of backgrounds will produce a variety of different looks.