I was recently working with a new assistant on location making portraits for a corporate client, when the topic of changing the look of the background came up. I explained to her that our middle-gray background (lit with a focused spot from a gridded strobe) could be turned into anything from white to black, simply by adjusting the amount of light on it. Then I showed her how to do it. The photos you see here are the ones made that day as I ran through various adjustments to the background light, demonstrating how the background illumination would change independent of the subject exposure—because you can always reinvent the background with changes to the background light, while leaving the key light on the subject untouched.
For starters, allow me to point out that what makes the changes seen here special is that they aren’t done with in-camera adjustments at all. That’s crucial if you want to control the background exposure independently from the subject exposure. All of these shots were made at ISO 100, at 1/125th of a second at f/7.1.
The technique is fairly simple, when you think about it. For the middle gray exposure, the power output to the background light was set at 125 watt seconds. (A watt second is a measurement of strobe power, equivalent to a light bulb of X watts illuminated for one second.) By adjusting it to 250 watt seconds, one stop more powerful than the original illumination, the middle gray background turns one stop lighter: light gray. By setting it to 500 watt seconds, two stops brighter than the original, it becomes white with detail, and at 1000 watt seconds (three stops brighter) it would become pure white.
Unfortunately, I couldn’t get to 1000 watt seconds, which is part of the challenge of adjusting illumination this way. At this point, I had to go another route: adjusting the light’s distance to the background in order to make it appear brighter. Moving the background light closer to the background makes it brighter—obviously—and again it does this without affecting the key light on the subject.
The reverse is also true. Dropping the background light output from 125 to 63 watt seconds made the background dark gray, and at 32 watt seconds it becomes very dark, basically black with detail. At 16 watt seconds it would be completely black, but I couldn’t get the power on my pack quite that low, so I again had to adjust the distance of the light to the background. This time I move the light farther away and it dropped the exposure accordingly. Alternatively, I could have added sheets of neutral density gel to the light to drop the output even further.
While these techniques are fairly simple and obvious, the results are quite dramatic. It’s the principle at work that I really want to hammer home. You can make huge adjustments to the look of your photographs by lighting each element separately, and adjusting light output and distance accordingly. It provides immense control over the look of the final output, and it works not only in studio situations like this but for all sorts of scenarios.
Exposure isn’t the only way to change backgrounds, though. You can also easily change colors with the use of inexpensive gels placed over the background light. This way a neutral gray background can become bright red or dark blue or whatever color you have gels for. And with the addition of a cookie through which the light shines, you can add textures from subtle to dramatic and really overhaul the appearance of your background. And, you do it all with nothing but light.
I think this serves as a great introduction for photographers who are new to working with studio lighting, and an equally important reminder for those of us with a bit more experience. You don’t need a different background for every whim; you just need the lighting skills to control background exposure separate from subject exposure. Then you can have a black background, a white background, or practically anything in between.