Conclusion: Impressive but can struggle with focus again difficult backgrounds, particularly if distance between subject and background is small. In the case below it held focus for long enough even though I was using f10 but there was a good distance to the background. Bear in mind I am a rank amateur when it comes to this kind of photography.
Original (@490mm, f10, 1/3200, iso1100)
Got lucky I guess!
I had trouble tracking these birds as they moved pretty fast. On the limit here with being able to track and maintain focus but probably a lot to do with poor technique on my part.
A long way off and lost them after a short while against this background. Still impressive that it could even pick them up at all – they weren’t really in the focus area either.
This bird was a long way off but the camera seemed to have no problem holding focus against a difficult background I would have thought.