Hi Rich,

The key is to make the background either 5 stops darker (if you want black) or the same intensity if you want a pure white background. The further back from the subject, the less light the background gets, thus darker.

If you are trying to do greenscreen/blue screen make sure the subject doesn't have any of that color in it and light it evenly. Different shades will cause you to open up the selection colors. Have to use 8/16 shades instead of 1.

Again if you want to create a separation, then have the background as far from the subject as possible, so as little of the light from the background spills onto the subject.