Canon 20D: Lens Auto Focus Constantly Adjusting Solution

The last few days I'd been having a 'problem' with my Canon 20D in that in the customizable modes (such as P/Tv/A/M) my Sigma lens would not auto focus properly. Every time I'd try and take a photo, the lens would constantly keep adjusting itself, jittering around and making a lot of whirring noises. Many of my photos were coming out unfocussed, so I ended up going to manual focus as I prepared to visit a camera repair shop, worried that all the bouncing from the amazing Faithless gig had broken my beloved camera.

Well it turns out it was a feature rather than an issue. At some point, most likely during the gig, I'd accidentally turned on the AI Servo feature. This changes the auto focus mode from where you half press the shutter release to focus once, to it constantly adjusts focus as you keep the shutter release button half pressed. Doh. This explained why when I went to full automatic mode the lens behaved itself. The simple solution? Press the Drive/ISO button on the top of the camera then adjust the AI mode to 'One Shot' using the front selector dial. Easy.

Of course now I know what the AI Servo mode is for, I find myself wondering why the Sigma lens seemed to go so crazy all the time - constantly re-adjusting its focus (from in focus to out of focus) even though nothing in the frame was moving. Sounds like that is a problem, but one that doesn't generally affect my shooting as I never use that mode.