How do you know if you need to do autofocus fine tuning?
Before I bought a 200-400mm lens, I never worried about fine tuning my camera bodies and any of my lenses. The camera manuals described the fine tuning process, but they said I would probably not need to execute the process. I took the manual writers at their word and did not bother to do tuning.
Soon after I bought my 200-400mm f/4 zoom I attended a Nikonians trek in the Tetons. During that trek, one my fellow shooters asked the trek leader, Jim Stamates, if he ever fine tuned his lenses and camera bodies. Jim not only answered with a firm “yes”, he illustrated the settings his camera had stored for each of his lenses. He also showed us how he did the autofocus fine tuning.
When I returned home from the trek, I tested all of my lenses using a manual test procedure I found on the Internet. I placed a yardstick on my dining room table, put the camera/lens on the lowest aperture for that lens, locked the camera/lens onto my tripod and shot the yardstick using a flash to ensure a good exposure (I later learned that using a yardstick in this fashion was not appropriate for AF testing…as I will discuss in another posting.)
After doing the test shots, I found that several of my lenses required tuning. The worst case was my 200-400mm lens. When I shot pictures of my target, the depth of field (DOF) for the shots did not center on the point that I focused on. The center of the DOF was behind my focus point…a focus problem called back focusing.
The attached photograph of the heron illustrates the issue. Although I focused on the heron’s head, the DOF for the shot centered on the back wing instead, causing the wing on the near side to be out of focus. By executing autofocus fine tuning, I was able to move the DOF forward for my shots with that camera/lens combination. Now when I put my focus point on a bird’s head and take a shot, the head is at the center of the DOF, and I have a better chance of getting both wings in focus.