> This does seem to be the preferred solution for modern autofocus cameras
> though.  Some use a CCD to look for optimum fine detail in the picture.
> Others have a couple of infrared windows in the front.  Do those use the
> main lens too, or is it an independent distance- measuring system?  How
> does it work?

Many such systems work on a principle similar to that of a split-image
viewfinder.  The camera projects a small spot of modulated IR and adjusts
the angle on an IR detector (or pair of detectors) so as to maximize the
pickup.  Typically, the rotating detector will be mechanically attached
to the focusing lens so that the act of lining it up will put the camera
into correct focus.