Focussing problems and general sluggishness

[Ruggerbugger]

Took my sigma and my 70-200 L to Wellesbourne today. I think I've junked 90% of the shots taken with the sigma, totally out of focus. Yes there's some distance to the active runway, but I was shocked at how poorly it performed. The thing is, I've got no idea where to start investigating, and I can't afford to just throw money at it.

[Bobster]

get hold of a 2x tele and see how you like shooting with it?