Originally posted by reh321 I know you included a "smiley", but this actually isn't a straight forward question. A faster object can be less of a focus challenge than a slower one is if the faster one is farther away, because the real issue is how fast is the percentage distance changing.
Ok so I did some measuring, 10 shots in 2.3 seconds and based on wingspan not taking into account that the plane wasn't perfectly centered (shouldn't alter the values too much) it was traveling at 855km/h (532mph) in the first shot it was 989m (3264 ft) away and in the 10th shot it was 443m (1462 ft) away and in the 11th shot it failed, beyond that it was to big to fit in the frame. This was using the Sigma 500mm F4.5 EX DG. This also doesn't take into account its altitude so its likely approaching even faster, maybe 600mph.
Using the assumption that the DPR bike test targets head is 20cm (10") ear to ear then the cyclist started at 32.5m (107 ft) away and ended after 16 shots (3.6 seconds) at 13.2m (43.5 ft) away. He was traveling at a blistering 19.3km/h (12mph). This of course at a 200mm focal length.
Interestingly each one cleared a little more than 50% of their distance during the burst. 59% for the cyclist and 55% for the jet. However the jet did it 1.6x faster and the lens focal length was 2.5x longer... however a 200mm lens would have to move further in the focus throw over the bike change of distance than the 500mm lens. The percentage of distance change can be described as 59% in 3.6 sec compared to 55% in 2.3 sec... the percentage of change win goes to the F-18. However I suspect the focus throw distance change goes to the bike test at likely 2-4mm vs 1mm or so of the 500mm lens, however throw time into the equation and the change in focus throw gets closer to even.
Another way to look at this is the jet was traveling at least 44x faster at a starting distance of 30.5x further away and don't forget the extra 2.5x the focal length
Take from this what you will, but the K-1 II can effectively track a very fast moving target coming towards you at a realistic distance away, as can many other Pentax cameras though maybe not as effectively.
Have fun!