There are occasions when a dark or even jet-black background would look good, but you are in a well-lit location. Perhaps you want to shoot a portrait but the background is distracting, or you want to make the subject standout from the background in a dramatic way?
The simple solution is to tell your camera that your subject is very brightly lit and make sure it exposes for just the subject. The background will then be under-exposed whilst the subject is exposed correctly.
Here’s how to do it.
Set your ISO to the lowest value possible. That’s usually ISO 100.
Set up your flash gun, and set the shutter speed to whatever the sync speed is for your flash (often 1/60 or 1/200).
Focus on the subject and if not already in manual mode, switch to manual so that auto focus won’t change anything after the focus is set.
Adjust the aperture until the shot seems totally dark. This usually means using an f-number at the high end, f/18, f/22 or above.
Take the shot with your flash on full power. The result should be a correctly exposed subject with a black background. If the subject is over-exposed, reduce the flash power and try again until you achieve the result you want.