How do I get a true black background?

[PhotosbyChuck]

To flag the light, you need to be side-lighting (not all the way to the side, but the light can't be coming from close proximity to the camera). Think of it this way: If the shadow from the model is falling toward the center of the background, you can't flag it easily as you'd need a cutout that's the shape of your model.

Of course, you could move the camera instead of the lights!

[CaptainKen]

I will add having you lights at minimum power will only change the f-stop for exposure and consequently the DOF, once you find the right formula of light distance to model and distance to the background you can turn your lights to full power and still get a black background. I normally shoot at 1/250 and f11 that way no ambient light finds it way into the shot.

[lvthunder]

I wouldn't want to put the light at full power. To get the background black I had to place the model 8' from the background and the light about 3' away from the model. With the porty on it's lowest setting I'm around f8. If I put it on full power I think my model would go blind.