[Bob Cole] “With Face Detect, can you select a particular face and have the camera track it, changing focus as that one face moves closer to the camera? If the person walks behind an object for a second, will Face Detect hold its fire and keep the focus at the setting where the face disappeared from view?”
You can pick the face you want to use. It will highlight all faces it detects and you use the joystick to swap around which face is targeted. Unfortunately, if it loses the face, the DPAF resorts to the adjustable targeting reticule, and it currently does not target where the face was last detected. So if you had targeted in the lower third of the frame, and the person whose face you are detecting turns their head to talk to someone behind them, it will immediately switch to the DPAF target and focus on whatever is there. I have asked Canon to fix this so that the DPAF target is adjusted to be where the last area of face detection was, but this has yet to be implemented.
You can tell the C300 Mark II to only use Face Detect, and so it does nothing when it loses the target, but then you’ll have to disable AF and go manual if you need to focus on anything else. And to switch it back to normal DPAF operation, you have to go through the menus, so it’s not ideal.