I just returned from another trip to Bosque Del Apache, and as one would expect the bird photography was fabulous. With thousands of cranes and snow geese flying overhead, I decided to run another test of the autofocus patterns using the D850. I do this test with every new Nikon camera I get since technology, autofocus and improved processing speed all affect performance.
Ever since seeing the amazing performance of Group-Area autofocus on a Chilkat eagle trip, this pattern has been my default choice for birds in flight. But I had heard the new autofocus modes were more effective at capturing moving subjects like birds. As I list below, a lot depends on the actual bird and where it is flying. A slow moving sandhill crane can easily be tracked in most modes, but a quick moving lesser goldfinch flying through the bushes is a whole new issue.
The short answer from my testing is simply this; for most moving birds, I still will choose Group-Area Autofocus in AF-C (continuous focus) at CH (fastest frame rate of my D850). Group-Area Autofocus gives you a pattern of five active focus points, a nice sized target area, to acquire your moving bird. I found this worked great on blue skies, birds flying past distant cottonwood trees, and even in dense brush as long as I could track my subject. Group-area autofocus works by activating all the focus points at the same time, giving you a larger focus area. Dynamic autofocus patterns only activate one point at a time (with additional sub-focus points depending on mode..i.e d25, d72, d125), so while they may cover more the viewfinder (and search for subjects), they don’t actually give you as large of an area of active main focus points to acquire your subject. Also, group-area autofocus gives priority to what is closest to the camera, resulting in less focus hunting than with dynamic modes. And finally, because of the larger active autofocus pattern, group-area autofocus sticks to your subject better than dynamic modes. I’m sure there are situations where dynamic autofocus will work great, especially with an erratically moving subject, but I still got my best results using Group-Area autofocus.
What about Dynamic 3D autofocus tracking? With this mode the camera actually moves the focus point for you trying to track your subject. This works great with subjects on distant backgrounds like blue skies, but is much less effective with busy backgrounds and cluttered scenes. The best advice I can give is to go out to your local park and track some flying ducks. Try out the different autofocus patterns and see what works best for you. In the end you will become more proficient with your camera and panning technique, which is what really helps you capture those birds in flight.