Digital photography

Instead of using a half-press of the shutter to autofocus, you use another button. By separating it from the shutter you can recompose between shots without the camera re-focusing every time you take one. Takes a bit of getting used to but becomes second nature after a while. Can do likewise with exposure.

Yes, or - more useful IMO - using AF-C, you continually track your focus point; the camera makes constant tiny adjustments, so you just need to hit the shutter button when ready.

Ewan, sorry to hear you're having this issue, but I promise you the lens was working perfectly on my D700...