How to measure light in black-and-white film portrait photography to get a black background?

First of all, you must ensure that the subject has enough exposure, otherwise the subject (such as human face) will be dark even if the background is dark. In general, when shooting, except for the dark background, we all use lights to suppress all background light. For example, gymnasiums and theaters are relatively empty in the back, and the lights can only hit people, so the back is naturally dark.