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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!

Cropped

100% Crop

@700mm
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.