Originally Posted by neuroanatomist
As Daniel Browning has pointed out, the 24-105mm f/4L IS on FF is better spec'd than the 17-55mm f/2.8 on 1.6x - the FF equivalent of the latter is 27-88mm f/4.5, so by comparison the 24-105mm is wider, longer, faster, and still has IS. True, faster shutter speeds are possible with f/2.8 (since the crop factor applies to DoF but not to exposure), however, the better ISO noise performance of FF compensates for that as well - higher ISO gives you a faster shutter.