Any idea about sweet spot of Sony 55-210?
I don't use mine that much, to be honest. I have a 18-105 and it covers most of my tele needs, and for portraits I use legacy 50mm and the "Beercan" (Minolta Maxxum 70-210/4). So 55210 is basically for travel, zoo, or when a friend invites kids over for some tubing (it's surprisingly good at tracking).
I try to keep it around f8-9, and so far am pleased with results. There seems to be some copy variation; some people say that theirs is bad fully extended, but then I think mine is OK. This shot was done at f7.1 /210mm, so almost wide open and at the extreme end, plus it's cropped, but I think it still came out ok.
The biggest issue with 55210 is available light, it's just too slow at the long end so it pushes you into high iso if the light isn't good. Also, I personally find the bokeh not as good as one could expect from a tele lens, at least when there are out of focus lights. But it's a very sharp lens with good stabilization and very portable.