The Canon EOS Rebel T4i (also called EOS 650D) is the first Canon SLR to feature autofocus during video using "hybrid" autofocus - a combination of phase detection and contrast detection technologies. With most SLR's, phase detection is only available with the mirror in the down position, but the T4i, with phase detection sensor built into the chip, can use it with the mirror open, i.e. during video. In theory, this is an important addition - using phase detection, the camera knows which direction to go, meaning less focusing "mistakes". How does this work underwater? I got to take the camera on a few dives, and here are my thoughts.
I found the performance of the autofocus during video very promising. With the Tokina 10-17mm and it's old school screw drive motor, the autofocus is pretty slow, but given a reasonable amount of contrast in the frame, quite accurate. The lens will occasionally breathe or hunt, especially when contrast gets low, but I found it on par with some dedicated camcorders that I've used in the past. I've got more work to do with this camera to try more lenses, and dial the settings in and find out what works the best, but the combination of workable autofocus along with Canon white balance makes this camera very high on my interest list.
The camera can be set to autofocus continously, or it can be set to autofocus only when the shooter presses the '*' button (BTW, the '*' button is easily accessed with the right thumb on the NA-650D housing). I found the later to be excellent for many shots... this is the equivalent of what we call thumb focus for still shooting - it only tries to focus when the shooter wants it to. This is very useful on a lot of video shots to prevent hunting, and opens up several possibilities for creative focus. Note that you can also do this with other Canon SLR's, like the 5D Mark III or the 60D; however, the big difference is that with other SLR's, the camera will typically open up the aperture while it tries to focus - meaning the exposure will be off. Not so with the T4i, meaning that the focus action itself might be useful in the final edits. This can actually be taken a step further, as the point of focus can be moved while shooting and refocus easily done with the thumb. This can be used to do "focus pulls" where subjects come into or go out of focus to add dramatic effect.
The autofocus motor is constantly busy and the built in microphone on the camera will pick up the sound of the motor. Canon released a new type of lens equipped with stepper motors (designated STM by Canon) along with the T4i that offers quieter focusing. Examples include the EF 40mm f/2.8 STM and EF-S 18-135mm f/3.5-5.6 IS. Lenses with ring ultrasonic motors, like the Canon EF-S 10-22mm or Canon EF 16-35mm f/2.8L II are relatively quiet, but lenses with screw drive or micromotors are quite noisy. Realistically, for any shots where sound is important, an external microphone/hydrophone and either an ultrasonic lens or STM lens would be called for.
Here is a short video clip showing the autofocus in action: