For portraits, it’s not unusual for photographers to employ a white background. After getting their Canon 5D MK II, it’s inevitably one of the first one or two backgrounds most photographers purchase.
You might think it’s easy to light a white background, or wonder if you need to light it at all. You will need to light it and it may be harder than you imagine. Once your subject gets four to six feet from the background, the light from the key falls off in a hurry. At six feet there can be a whole stop difference between your subject and the background. At a stop under the background is not going to be white, it’s going to be a flat gray “vampire background” that sucks the life right out of your portrait.
Situations like these are why incident light meters and flash slaves were made. You’ll want a softbox or umbrella on each side, positioned four to five feet off each side of your background, usually off camera behind the subject. Use a flag or white panel to keep the background flash from highlighting and outlining your subject. That will not be a pleasing look.
Adjust your flash power until the background is a stop lighter than your subject. That will give you that nice pure white glow without blowing back on your subject. In the video Gavin Hoey suggests two stops, but in my experience one is enough unless you have a lot of flaws and wrinkles in the background you’re trying to hide.
Take your light meter and check the back of the subject, just to make sure you’re not getting highlight from the background. The meter check behind the subject should not be any higher than in front. If it is, move the lights or your subject farther away.