When you use an f/2.8 lens on a camera that has a high-precision f/2.8-sensitive AF point, you get the benefit of that high-precision AF point that you don't get with a lens slower than f/2.8 ). This is specified by Canon as 'accurate to within one depth of focus' for normal precision and 'accurate to within 1/3 the depth of focus' for high precision. AF systems aren't perfect - any given shot will be within some range of 'perfect' - using a lens that enables the high-precision point generally means that more shots, on average, will be closer to 'perfect'.
Yes. Thanks. That was exactly what I was referring to.
So... even if I was shooting that f/2.8 lens at a narrower aperture (f/4 or f/16) the "precision AF" would still work to improve shots on that lens? Or does the precision AF only kick in when you actually dial in an aperture f/2.8 or wider?