I was wondering if anyone knew whether it was possible to control the focus of an AF lens on one of the modern cameras like the 5D2. And, if so, is there an API such that I can control the focus from a programming language like C/python? Any help would be much appreciated, thanks!

By control the focus I mean, I want to tell the lens what distance to focus at, or to literally just control the USM motors, whatever to that effect is possible.

I don't know for sure, but Canon is not exactly third-party friendly. I really doubt they would publically document how to control their cameras/lenses.

Maybe checkout this site:

http://magiclantern.wikia.com/wiki/Magic_Lantern_Firmware_Wiki

They've been tinkering with 5D2.

Yes, this can be done. You need to register as a developer with Canon and you can get a Mac/PC SDK. It's pretty poorly architected but it works. I use it for night exposure control and a buddy at work uses it for focus control (helican focus or whatever) to get more DOF for macro. You write code in C / ObjC on the Mac side - I don't know / care for PC

Awesome, thanks guys!

kewlcanon already mentioned Magic Lantern, but I thought you might find this link particularly useful:

http://magiclantern.wikia.com/wiki/Focus