Canon T2i - Live View vs. Quick Mode

I have an Ikelite housing for my T2i and the Ikelite manual recommends using Quick Mode.

If using auto-focus in “Live-View”, use “Quick-Mode”.
NOTE: Auto-focus will not be as good in “Live-View” mode.

Based on my limited testing I have not found any issues with Live View mode. I was wondering if anyone has any comments; good, bad or otherwise.

Live view cannot use the phase detection focusing system because the mirror that bounces a portion of the light to the detectors are flipped out of the way. So it has to use contrast detection focus system that are standard in point and shoot cameras. With phase detection focus, the camera knows which way it would have to adjust for the image to come to focus, while on the contrast detection doesn't, so it frequently has to focus in one direction and if it realizes the contrast is decreasing, focus the other way.

Also, phase detection allows the camera to follow a subject's focus because once it moves out of focus, while for the contrast detection, it just knows the subject has less contrast and will adjust the lens in one direction or both until the contrast level is highest again (thus subject is in focus).

ccredifo,

To summarize Lwang's accurate description, autofocus will still work in both modes. Phase detection AF is the quickest, but since it can't be used in "Live-View" mode we recommended "Quick-Mode." This proves to be most beneficial when shooting moving subjects.

Regards,