I try to keep the lens in context, it's not a super fast lens for action. It's just not. The aperture and speed of focus should be the biggest indicators that this lens was not really meant to perform when it comes to things like birds in flight and capturing any sort of fast moving action the way an F4 or F2.8 telephoto with USM does. The Tamron really is basically just a telescope, meant for relatively static stuff, some minor action, but not the extreme things. Sure you can do some of the more extreme stuff, with good skill and a little luck, but ultimately again this lens wasn't built for that. There's a reason it has really nice, sharp optics, and 600mm, for only $1k, instead of being a $6k+ lens.