One of these days I should invest in FoCal. I just haven't been dissatisfied with any lens enough to push me to buy the software. For a quick test I line up three water bottles along the countertop in my office and shoot them (tripod mounted) from 15ft or so at an angle. I focus on the center bottle, of course, and see where the sharpest area is in relation to my focus point. On the 7D I made a few AFMA corrections this way. I haven't had a need to do so on my 6D yet.