Originally Posted by rowan

I just had a quick look at the lens list and most of the L series telephotos are the same aperture at the wide and telephoto end of zoom range... the only exception seems to be the 100-400mm f4.5-5.6L IS USM

One other issue is where focus changes as you zoom, although that should be quickly fixed if you're in AI servo mode. (Even if you're not a quick half pump of the shutter will refocus...)

From memory there are several L lenses that are parfocal which means they don't need to be refocussed if they're zoomed.