In article , Ben Micklem
wrote:

In Live View mode, it will use contrast detection AF on any desired point in
the frame (i.e. change the focus, see if this increases contrast, if not, go
the other way, etc.). All other AF modes won't work, as the focus sensors
are in the bottom of the mirror box, and they get their image from a
secondary mirror that reflects light from the middle of the frame (that has
passed through the semi-silvered primary mirror). As the mirror is up during
live view, this can't work. I think there is an Olympus live view method
where the live view comes from a second colour CCD in the prism housing, and
so the mirror is down.

the nikon d3/d300 live view has two modes. in handheld mode, the
mirror will drop and autofocus will use the usual 51 point focus
sensors, then the mirror goes back up to resume live view. in tripod
mode, contrast detection is used off the sensor itself and the mirror
does not drop to focus, with the focus point placed anywhere in the
frame.