James Colwell , Nov 12, 2008; 06:23 a.m.
You might correctly remember hearing or reading that "the AF system works of a range of 15-20 stops", but this is not relevant (and it's probably not correct). Autofocus is performed with the lens wide open. That's why you have AF problems with extension tubes. Also, your intuition is wrong - 1 stop is a large difference, especially for the AF system.