Sun 26-Aug-12 04:51 AM | edited Sun 26-Aug-12 12:35 PM by aolander
Think about when taking a photograph of any scene (no flash). The faster the shutter speed (aperture staying the same), the more underexposed (darker) the image gets. It's the same thing when using flash for the subject and ambient light for the background. The subject will be correctly exposed by the flash at any shutter speed (at or below the sync speed), but the background will become progressively darker the faster the shutter speed.
When the lights are dim, it will be quite easy to make the background black because you'd have to use a large aperture and slow shutter speed to expose the background correctly. For example, if the room lighting would require an aperture of f/2.8 at 1/30th for a correct exposure, shooting at f/5.6 and 1/200th with a flash would underexpose the background by 5 stops.