[Ian Stuart Forsyth]

When we are evaluating how well a camera can AF in an action sequence we always have to consider the context to the challenges for the AF. Some of the more important aspects we always have to consider is the relationship with how fast the subject is moving , how much DOF we have for that scene and how long it takes that subject to travel through the allotted DOF. Looking at only these 3 (of many) parameters we can obtain a better understanding of the complexities for the AF to predict and place the focus plane close to our target with accuracy.

Just focusing on Z tracking and ignoring X and Y tracking that uses the metering sensor, we can look at the what three above variables play. We can take a look at 2 scenarios , trying to track a person walking at 5kmph towards the camera 10m away using a 200mm lens at f2.8. The second being a bird in flight traveling 50kmph with a 600mm at F6.7 at a working distance of 70m.

All we have to look at is the size of the window(DOF) and location (time)the camera is trying to place the focus plane into. For a bird traveling at 50kmph that window is 5.5m wide it takes the bird 0.3 secs to travel thru that DOF.
With the person walking we have a window that is 0.42m wide and takes the person 0.3 secs to walk thru it, that is somewhat smaller window for the camera to predict and place that focus plane into.

Now with the man running at 12kmph that window is still 0.42m wide but the time the runner stays in that window is only 0.12 sec an extremely more challenging with a narrower time to place the FP in.

Now lets take a look at just the time frame and distance traveled for the BIF, most cameras that can do 8fps (1 frame every 0.12sec) that camera will only need to adjust the focus plane on every second shot to keep the target in the DOF and this will show up as less missed shots, with the runner the camera will need to adjust the focus plane for every shot to keep the runner in the DOF window this increases the accuracy needed to keep every shot in focus.

Without the context to the challenges a cameras can be said to track a target traveling at (3400km) and never miss a frame, but then again almost everyone can MF on the moon