OK, so unfortunately I sprained my ankle last night playing football, so could only be bothered to complete part of the test I was going to carry out. I didn\'t want to fall over with my lovely new lens!! I tried both Sigma 150-500 & the Tamron 150-600mm on 7D body. I kept my thumb in my AI Servo button (which I\'ve pre-programmed to my \' * \' button) throughout both tests and can confirm the Sigma kept focus from near, through mid to far distance without issue. However the Tamron didn\'t. It stopped when it got to the mid distance and wouldn\'t pick up focus (or hunt) again on the far distance until I took my thumb off the button and pressed it again. This was carried out at 400mm on both lenses. I\'ll upload the results once I\'ve found & uploaded my DPP software, which I understand shows the focus points? Hopefully someone else with this lens can try a similar experiment and see if they get the same result?
Diverhank, sounds as though you have issues too, that doesn\'t sound right to me. The whole point of AI Servo is to track. Admittedly if you\'re tracking through \'challenging\' scenery in difficult light then the odd b(p)ump or two would be required, but twice a second every time sounds excessive to me - especially with that set up!! I had a 2nd hand 30D with a £600 Sigma lens that didn\'t require that amount of effort!
