Thanks for the replies. Surf is big today, but raining, so no practice for me..
The D810 has the 3D mode, so that might be worth trying out.
Regarding the 9 dynamic focus points... I might have to look into that more. The manual is a bit scant on the details of how this option works. But maybe if I concentrate hard on only half depressing for focus when the centre point is over the surfer, it might work better. Is that definitely how the dynamic focus mode works?
Instead of shooting today, due to the rain, I decided to look into a few other lens options. I'm a little short at 200mm (300mm in DX crop mode). And as I'm often shooting towards the end of the day, speed can become an issue (even though it's an f2.8 lens). I'm wondering if it is worth paying a few more dollars and sourcing a VR - optical stabilisation - lens. But I'm unsure how the VR/etc will work while panning. Does anyone know if image stabilisation works in a panning situation (i'm usually on a monopod), or does it just mess up?
edit: I also notice that some of the sigma lenses have two OS settings. What's that about? From memory of my previous canon gear, I think one of my lenses had two stabilisation settings. One of them might have been for panning/tripod??