I’ve been experimenting with the spot autofocus mode. As far as I know, Canon introduced the Spot Autofocus with the Canon 7D and ported it to the Canon 1DMk4. The spot focus is available on all the 19 auto focus points. You can even select which spot focus point to use with the joystick.

According to Canon:

A couple of important notes about Spot AF:

  1. While the AF point size is definitely reduced compared to ordinary Manual AF point selection, it’s not the tiny square you see in the finder. The actual area being sampled by the AF sensor is larger than the inner box, so allow for that when composing and shooting.
  2. Also, dual-axis, cross-type sensitivity remains at all AF points if you select Spot AF mode, so there’s no loss of effectiveness there.

— Canon US

The bold is mine and not Canon’s.

I’ve found that:

  1. Focusing tracking is much more difficult and much poorer. Even though the subject is bright and contrasted, I have significant problem keep the spot focus on the exact contrasted area of the subject and having the right area of the subject to be focused. Happens a lot to me when I use the spot focus to focus on the eyes.
  2. Focus tracking seems to be much slower than with the single point autofocus.

I’ve come to the conclusion that, for me, the spot focus is mostly for static subject. There are too many ways of screwing up with moving subjects.