The focal length you use would depend upon how far away you are from the stage. If you are not too far away, the 50mm f/1.8 would be good. An f/2.8 zoom would work, too, but here again the focal length depends. A 24-70mm f/2.8 if you're close, a 70-200mm f/2.8 if you will be further away. Either will cost you some bucks, though!