I am currently using 17-40L F4, 70-200L F4 and 50 F1.8 on 300D. I found myself like to shoot with the tele lens more. However, 70-200L is not long enough sometimes, and adding 1.4x TC will further reduce the f stop. So, I am thinking to get 300mm F4 IS to replace 70-200. Meanwhile, there's a...