When you are shooting a zoom lens that is F2.8 all the way through the range of the lens, your camera will focus fast due to focusing at the widest aperture available of F2.8. So if I am shooting a lens that is say 18-55mm F3.5 to 5.6 does the camera focus at 3.5 all the way through the zoom range or does it change as you zoom out and the F-stop changes to f5.6 with the longer zoom? I just want to get this clear in my mind.