This photo was shot against a gray background. At the last minute, the client said they wanted all the shots made against a black background. No problem. We just moved everything back away from the gray background to make it look black!
Here’s a quick tip about flash and how it impacts your background.
If you have a white background, you can make it appear white, black or any shade of gray in between just by changing the distance of your flash from the background.
The closer your flash is to the background (assuming you use use your flash set to manual mode and use the same flash power and the same shutter speed for each shot) the whiter the background will be in your shot. The further the flash is away from the background, the blacker the background will be in your shot.
Why is this? It’s called fall-off. Light falls off because of a thing called the inverse square law. What’s this mean for flash photographers? An object (like your background) that is twice the distance from a flash head will receive a quarter (1/4) of the illumination – or two stops less light. An object that is three times the distance receives one ninth (1/9) the illumination – or three stops less light. An object that is four times the distance receives one sixteenth (1/16) the illumination – or four stops less light, etc.
The lights spreads quickly and because of this spread, a smaller amount of light hits the background. As less light hits the background, it gets darker.
Try this experiment at home. Set up on the whitest background you can in your house, garage or studio. Set your flash to manual at one half (1/2) power. Set your shutter speed to your maximum sync speed (will run between 1/60 and 1/250 of a second depending on your camera) and pick any aperture that will give you a decent exposure. Then take a picture of the white background from three feet away, then six feet away, then nine feet away, etc. Look at the results.
This post is simply designed to teach you the basic concept of light fall – off. It’s not meant to be a white paper on the subject. I’ll go more in depth later after establishing some more basics.