Reader Paul Boyce finds that the iPhone 4S's camera is inconsistent when it comes to autofocus. 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.
We passed this one around the office. Both Dan Frakes and I have an iPhone 4S and there's something in what you say--sometimes. In our testing we didn't find it to be a consistent problem. For example, the first time Dan tried it, his iPhone auto-focused perfectly. However, I gave it a go with my iPhone--pointing it outside to focus on a fence and then pushing a printed page in front of the camera, much as you did in your video--and the phone focused only when I tapped the screen.
We each took another stab at it. This time Dan's phone didn't auto-focus correctly. With my iPhone I tapped and held on the screen to lock focus and expose, tapped it again to disable that feature, and tried again. Auto-focus worked. On a third attempt under the same conditions, auto-focus eventually worked. The phone focused when I pointed it outside, again when I placed the printed page in front of it, but took about four seconds to re-focus when I pointed it outside again.
So yes, there's an issue, but its consistency can't be counted on. In the meantime, if you really need to focus on a subject and your iPhone seems incapable of it, give the screen a very gentle tap to focus.