by C-M-B » Wed Jul 18, 2018 9:19 am
The AF of the 645DF+ is "okay" but not really made for fast moving objects, especially not objects that move towards you or away from you at a high speed.
That being said you can still manage such shots quite well by proding the AF with a good contrast between subject and background and/or by working around the present limitations.
If for example the model is walking towards you, you'll need to either get the image in 1-2 shots or move back to keep the model in frame.
In the first case you can set it up so that the AF doesn't have to do a lot of hunting before you press the shutter (the model will only be able to walk a few steps before he/she is out of frame anyway). In the second case the AF won't have a lot to do except for compensating for small differences because you'll have to keep the distance somewhat constant during walking.
A lof is focussing problems are also related to the lens you're using and how fast/slow its AF is. If you're using a tele-lens in a dimly lit studio the AF will have a problem finding its target - and with 2.8 it's really hard to get it in focus at all and also very hard to keep it in focus.
The 645DF+ only has AF in the middle, so most of the time you'd have to focus and then reposition the frame for a composition - with f2.8 that's often enough to change the DOF and you'll lose sharpness in the eyes. Besides, f2.8 for headshots is pretty extreme, that's hardly enough for keep a whole eyelash in focus for headshots.
It's hard enough to get it done with f4.0 - but if I manage to do so, it's only with a tripod or Monopod on manual focus.