I've had an Nikon d3500 with the 18-55 and 70-300 VR kit lenses for about a year, and until recently the autofocus has works as I expected (I almost always use single point AF and AF-C with back button focus). But now it has a weird problem where the central af point works weirdly. It hunts endlessly and it always misfocuses. Sometimes it just goes to minimum focus distance and stops trying to focus. It is only really able to focus reliably on really bright objects, like lights. I'm pretty sure the problem is in the autofocus sensor itself, as all the other points still work normally.
Also, when looking through the viewfinder, there's a small weird circle around the middle af point where the image looks a little but brighter than the rest (It's kind of hard to describe it, I can try to photograph it). It has always been there, so I don't think it's related, but I though I'd mention it.
My guess is that there's some dirt in front of the af sensor, but I've tried to «shake it off» (since I don't have a blower) but it's still the same. Any ideas on what it could be?
I hope I was clear explaining the problem. If not, please say it so I can try to explain it better.
Thanks in advance!