Thought this will come in handy to those who battle with soft images and or focusing.
It helped me with my 40D focusing and explaines why image sharpness improves if you activate the focusing system more than once. The phase difference calculation is an open loop system, which only looks once at the beginning of the process. The minor adjustment that follows is a closed loop system. If the initial adjsutment is a large one from way out of focus to sharp, then the random focus selection in the sensitivity range of the lens may result in softer enlargements. Canon says sharp is 6x9 inches viewed at 10 inches.
Hence Pixel peeping may = soft
It takes a while to study this system and how to get the maximum out of it.