I am shooting a Canon 40d. close lens are 28-135 IS and Tokina 12-24/f4. I want a closer Zoom or Prime...say the Tamron 17-50 or Canon 35mm/f2. My dilemma is this: I could get a cheap prime/Canon 580 EXii speedlite for the same price as a good 2.8 Zoom. This would seem a no-brainer to most...