Quote Originally Posted by Squidy View Post
So, I guess the question from me is, what's the difference? Why would the camera offer only so many focus points through the viewfinder but offer so many more in live view?
Viewfinder focusing uses phase-detect AF as others have noted. The main mirror is translucent; some of the light passes through the main mirror and then reflects off the secondary mirror down to the AF sensor. There's some beam-splitting magic going on so the light arrives onto two different places of the phase-detect AF sensor for each "point", allowing the phase differences to be checked. These phase sensors are very task-specific, as they can tell not only IF focus is achieved, but if not, they know WHICH WAY TO GO to achieve focus.

Live-view focusing uses contrast detection techniques on the main imaging sensor. The selection box defines which pixels are sampled for contrast. When you request a refocus, the AF motor "runs the gamut" from <here> to full-infinity to full-macro, sampling the contrast as it goes. Once it's done a full sample, it (should) return to the point that had the most contrast. As such, it's "very easy" for the contrast-detection "action" to select almost arbitrary pixels for use as the CD region.