No problem with low light focusing on my 1ds Mk3
that I can find. Works fine.
One thing I do notice from time to time is that the center red focussing
point will show, not one, but two focusing points when set to one shot mode. I traced it to a bit of rotational play between the camera mount and the lens itself. If I slightly rotated the lens one way to take up the play, the two focusing red zones did not show up, just the single one.
Rotated the lens in the opposite direction and, bingo, the two red focussing zones show again.
I've only tried this with my 35mm fi.4 L and I've yet to try it out with my other lenses. It could be the camera or it could be the particular lens I'm using?
Will report back when I've tried further lenses.