not sure what a/m is but when it's concerts and performances in general you're a lot better off with nothing higher than f/2.8, from personal experience, unless the lighting is damn power (which has happened to me once from experience, i was using my 18-55mm at f/3.5) if you're next to a barricade in front of a stage, 17-55mm for a zoom is good, so is 50mm. mid-range like 24-70mm if you're standing somewhere in the middle of a crowd, long range if you're standing right at the back.. but it all differs with venue. you could try working with flash, but places like esplanade don't allow it, even outdoors (you can try tho, i've heard of people getting away with it)
tips... know what kind of performer you're shooting. watch their movements for a bit, then try at capture them at their highlights. don't always snap away, just keep waiting for the right moment. i have experience with shooting rock bands and if you know the song that they're playing, the part they call the 'breakdown' is always something to watch for. a lot of this comes with experience. check out
http://ishootshows.com/2008/07/16/choosing-lenses-for-concert-photography/ , great site if you're shooting concerts. good luck and hope this helps!