The origin of this question is related to panning. I was trying to explain why panning is more effective for subjects moving across your FOV at right angles, vs. a subject whose distance to the camera is changing -- namely, once an exposure starts, focusing is locked for the duration of the shot (as far as I know).
It occurred to me, though, that there are a couple possibilities for focus-following during exposure (and probably some I haven't thought of), including:
- AI focus (Canon) -- as advertised, this is "predictive" autofocus, so I'm not sure why it couldn't keep predicting during an exposure
- Live-view focusing -- probably more difficult, technically, because the camera would have to switch focusing methods as soon as the exposure begins and pick up the tracked subject. Might be easier with something like Sony's translucent mirror.
In any event, I'm unaware of anyone offering an actual feature like this today -- is that right?