Reader Paul Boyce finds that the iPhone 4S’s camera is inconsistent when it comes to autofocus and shooting video. He writes:
There appears to be an issue with the iPhone 4S video camera that affects my 4S as well as that of two other friends who have the same phone. The auto-focus function on the video camera does not appear to re-focus as it should. I’ve posted a YouTube video that demonstrates the issue.
As it turns out, what you're seeing is the way the iPhone's camera is supposed to work. Unlike the still camera, which does use autofocus to pinpoint your subject, the movie camera doesn't rely on autofocus—if it did, you might see wild shifts of focus as you move the phone around. The intended way to use the movie side of the camera's function is to point the phone at your subject, tap on the subject to focus and get the proper light reading, and tap record. If you happen to move the phone in order to focus on something else, tap the screen lightly to attain that focus.
"But…but…" I hear you sputter, "my phone very definitely appears to autofocus before I take my shot." Yes, it does. It's making some effort to figure out where the subject is but it's really up to you to tap on that subject so the phone knows exactly what to do. While shooting a movie you may also see some focusing efforts, but those efforts aren't really intended to lock in on your new subject. Again, if you want to get the best focus and light performance while moving the camera around while shooting movies, tap on the screen when the new subject is lined up in the frame.
Updated 10/28/11 to provide correct information about autofocus and the iPhone's camera.