Quoting from the 50D user guide : "With lenses whose maximum aperture is larger than f5.6 : ...all AF points cross type AF ...is possible; With lenses whose maximum is LARGER than f2.8 : With the center AF point, high precision cross type AF sensitive to both vertical and horizontal lines is possible. The center AF point is...twice as sensitive as the other points". How come it doesn't say anything about the diagonal sensitivity that i've read about on the Web? And why does it say "larger" and not "equal or larger"? Now i have to buy a fast prime to utilize the center point's additional sensitivity?