Originally Posted by Marc Sabatella
The problem with flash in environments like conventions is that it can only illuminate a limited distance in front of you. So you'll get nicely exposed stuff a few feet away, but stuff in the distance is going to be dark. So the trick would be to be close to whatever you want to take pictures of and not expect much to be visible in the background. If you *want* the background visible, then it's time to turn off the flash and rely on available light, which will have the advantage of being more consistent throughout the room. Your lens is not going to be a great choice, since it doesn't have a particularly wide maximum aperture, meaning you'll have trouble getting fast enough shutter speeds. But your best chances will be t the widest end of the zoom range, both because it has the largest maximum aperture there, and also because you don't *need* as fast a shutter speed at wider angles. So I'd turn up the ISO as high as you can, zoom all the way to 18, shoot in Av mode, select the smallest f-stop number you can (probably 3.5), and see if the resulting shutter speed is one that has any chance at all of producing a sharp picture. If not, dial in negative exposure compensation to speed the shutter than expect to have to fix in in PP.
It is possible to use flash and still have ambient light backgrounds. Take a test shot and whatever the exposure, decrease the shutter speed. When the flash fires, it will get the person properly exposed, and the longer shutter will allow the ambient light to come in and brighten the background.
Some cameras have a flash mode called Slow Synchro that does this automatically, otherwise you have to do it in M mode.
One caveat is that most people will move as soon as the flash fires, so either prep the person, or set the flash to rear-curtain (or second curtain sync).