I was out photographing a cross-country race last night and had one instance where my lens would not track focus on some runners that were coming right towards me. AF tracking of runners is something that is very straightforward for the Nikon D750 autofocus system coupled with a Nikon 70-200mm f/2.8 lens. This combination is excellent for sports photography and should have done a great job in this situation.
I had the camera set for group-area autofocus, which means there are five AF sensors that work together in a group to track the subject as it moves. I also had the camera set for continuous autofocus (AF-C) and continuous high (CH) frame rate. I was holding the camera in the vertical orientation with the shutter release down towards the ground, while holding the lens with my left hand cradling the lens barrel.
After the camera didn’t track focus for one set of runners, I immediately looked at all my settings to make sure they were correct. Sure enough, all the lens switches were set correctly and all the camera settings were set correctly. Hmm.
My next troubleshooting step was to see if my lens was mounted correctly. Sometimes if the lens isn’t fully clicked in place the electronic contacts can prevent auto focus. I wiggled the lens a little bit in the mount and found that it was okay as well. Hmm.
So, what was causing the autofocus problem? Well, on the next set of runners that came through I held the camera in exactly the same orientation, looked at my left hand and noticed that my fingers were resting on the manual focus ring of the 70-200mm f/2.8. Aha! With a silent wave focus lens like the Nikon AF–S lenses, if you rotate the focus ring while trying to autofocus, then the autofocus system will stop and the camera will immediately revert to manual focus.
This is generally a good thing and I use this technique all the time when photographing wildlife. It allows me to autofocus quickly on the animal’s body, then I can rotate the focused ring manually to fine-tune autofocus without having to change my composition. In the case of the cross-country runners last night, I simply bumped the manual focus ring, which kicked me out of autofocus for a short period of time. I made a rookie mistake and I won’t be making it again.
The moral of the story is when holding an AF-S lens, make sure you know exactly where your hand is placed and don’t rotate the focus ring when trying to autofocus.
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