For a typical seamless background, for instance, many studio photographers use a single light source. It’s especially useful for spotlights and graduated backgrounds in portraits, but this approach falls a bit short when an even edge-to-edge illumination is desired. In these cases, rather than a single light centered below the frame, I like to use two lights off to each side. I position the lights in the vertical center of the frame, and I try to keep them as far from the background as is reasonably possible without interfering with the subject. Flags can help in this situation to block spill from the subject while providing distance from the background. It’s that distance that makes the illumination broader and more even.
When I can, I prefer to use umbrellas or softboxes to make the light as broad as possible. Parabolic reflectors are more likely to create hot spots on the background and corresponding darker areas as well. Two lights help to alleviate this issue, but diffusion helps even more.
With the lights positioned opposite one another, equidistant from the background, the final positioning step is to feather the lights across the background. Rather than aiming each source at the center of the background (which would create a hot spot), I aim the light on the right toward the far left edge of the background, and the light on the left should be aimed across to the far right side. This too will help even out the background. Now I’m ready to use a light meter to check just how even it is.
Using a handheld incident meter (the kind with the white diffusion dome) position the meter at the background and in the center of the frame, being careful not to block either of the background lights from fully illuminating the meter. Press the button to take a reading. (With strobes, you’ll need to trigger them in your preferred method—via cable, wireless or manually fired.) Take a reading in the center of the frame (let’s say it’s ƒ/8) and then move the meter to one of the corners of the frame. It’s helpful here to be able to see the frame (live view helps with this, for instance) or have an assistant guide you. Take a meter reading in that corner (say, for instance, that it’s the top right) and compare it to the center of the frame. It should ideally read ƒ/8 as well. Read the next corner (bottom right, let’s say) and it too should read ƒ/8, as should each of the other two corners.
If any of these corners reads more than one-third of a stop different, it’s going to be evident as uneven lighting in the finished image. A one-tenth-stop variation will be almost impossible to see, and a one-fifth variation—well, that’s a tough call. Sometimes it’s fine, other times you need to fix it. That’s when it’s up to you to decide what’s right.
If any of the corners read a little too dark, turn the light on that side of the frame slightly more toward the center or move the light slightly closer to the background and meter again. If any of the corners reads too bright, move the light slightly farther away. With this method, you achieve very even illumination across the background, ensuring it with the aid of a handheld light meter.