Don't most of the variable aperture zooms hit the smallest of the aperture range rather earlier than the max. focal length, like - say - the Sony 100-400mm f/4.5-5.6 GM becoming an f/5.6 lens by 162mm (as reported by Bryan of TDP in his review)? 
I know the Canon 100-400 II was a positive example though, only going to f/5.6 after 312mm, but it's a rare case.
I know the Canon 100-400 II was a positive example though, only going to f/5.6 after 312mm, but it's a rare case.