On mine it's ok, I can turn it most of the times with my thumb only and it's not ergonomically difficult to use two fingers while holding the lens with the left hand.
If it were looser it will move too much; I wish the aperture rings on their lenses were so tight, none of them really is and it doesn't get any better with time.
Ergonomics are trickier for the both the shutter and the ISO, it doesn't bother me that much because I use aperture-priority and auto-ISO most of the times, but when I try manual something it's not that fast or easy.
