I was asked how I kept the background in some of my shots as black as they are.
I use a black non-reflective background.
I use a big cotton sheet that is without folds or wrinkles.
I put the light near to the model and I put the model as far as possible from the background.
That way I don’t need much light to light the model adequately, and the light falls off quickly and doesn’t reach the background.*
That’s why I need a big background sheet. If the sheet were just behind the model I could use a smaller sheet, but when the background is far behind the model, I need a bigger sheet to fill all the space behind and around her.
I don’t recall the exact distances for this shot, but I guess the flash was in a softbox about five or six feet from the model, and the background was about ten feet behind her.
The principle is easy. Keep the light near to the model and keep the background further away. If you shoot digital you can shoot and look and see what the image looks like.
If you shoot film, beware of cutting down on the distance between the model and the background. You think it looks OK to your eye when you are shooting it, but ignore that rule and you will get caught out and the background will not be black.
You will see the shadows cast by the folds and creases in the background cloth. The cloth will appear grey rather than black.
The point is not to illuminate the background at all.
If you use a paper background you will avoid the problem of light that catches on the folds and creases of the cloth.
But paper rolls are not cheap and they are usually only used by very serious amateurs or professionals.
And provided you are shooting above the floor, you don’t need it. You do need it however, when you are shooting someone right down to their feet, because then you need something that has absolutely no folds or creases – and only paper does that easily.
By the way, this was shot on film – probably Ilford Delta 100 and the camera was either a Nikon FE or a Nikon F90x.
Light Intensity And The Inverse Square Rule
This law of physics tells us how quickly the intensity of light falls off with distance. Imagine the light fully illuminates the model who is five feet from the light. The inverse square rule tells us that the intensity at five feet is 1/5×5 = 1/25 and we made it so that’s enough to light the model.
Now if it’s another ten feet to the background, then the intensity of the light when it reaches the background is 1/15×15 = 1/225 which is 1/9th of the intensity of the light at five feet.
If we had put the model at ten feet, we would have needed to pump more light onto her, and that means more light would reach the background.
You know, the same kind of rule applies when we want the background to show, but we don’t want harsh shadows. One way is to diffuse the light with a softbox or similar. But it’s also a good idea to keep the model away from the background – unless we want moody shadows, of course.