Here is the setup I used for these focus tests. I've found that a flat target focus test setup is more reliable. The tests were done tripod mounted squared to the target (as best as possible). Mirror lock up, self-timer and central focus point were used. The lens was defocused before each shot. The RAW files were converted in DPP with 7 sharpening to aid in DOF determination. I didn't take multiple shots at each aperture because I've found the autofocus on the lens to be very consistent. That said the is the occasional misfocus so it is possible that some individual shots for these test could be off slightly. Note that my XTi was recently calibrated by Canon.