Two cheap lens compared. On the left, a Canon EF 50 II fixed focal length, on the right, a Canon EF 28-80 USM IV zoom. Both used at F4.0 in the exact same conditions, tripod, remote trigger, 550EX + lumiquest bouncer in front, slave 420EX bouncing on ceiling. The small size difference is apparently due to the fact that the zoom ring isn't extremely accurate. This being said, the difference is striking.