A few days ago I mentioned that I was considering calibrating the AF Microadjustment on my 5D Mk3. I started messing around with it and found that making that calibration isn't as easy as it seems. I tried a couple different methods and discovered that my measurements just were not very reproducible. I went to YouTube and did some checking and found an idea that made sense. The method was to photograph batteries sitting in a diagonal line with the small printing showing. At first I thought something was not working properly at short focal lengths (24mm). Things seemed to be working well with the long focal length (105mm).

So I sat and thunk for a couple days. Magically, my brain finally figured it out. The experiment was to line the batteries up diagonally on a table about a meter away from the camera. I shot at f/4 which would give me a narrow DOF. Well, the problem was that the batteries were pretty small in the frame at short focal lengths, so I had to enlarge them quite a bit in lightroom to have a good look. Sure enough that enlargement compared to the more modest enlargement at 105mm resulted in a less sharp picture, of course. Here you see the 105mm shot with focus set at the center battery. The other batteries are out of focus, meaning that the autofocus was spot on at the focal length. I need to try it again at the short focal length and get closer.