I just picked up a Sigma 17-70 2.8-4 OS.
It focuses lightning fast at 17mm, but when it is exteneded to 70mm (or anywhere semi-telephoto), the focus pulses back and forth at least 6 times before it finds focus.
I think this is unacceptable, and it might just be my lens. I can focus quickly on the same spot at 17mm, but if i try that same spot at 70mm it takes at least 10 quick back and forth pulses before it catches.
Has anyone else had this issue?