My Nikon D3100 out of the blue cannot focus correctly.
It seem the that the focus motor always stops at the end of the run (Infinity) and cannot lock on the target, thus cannot release the shutter. I tried with two other AF-S lens but still the same.
This only happen when I use the viewfinder. The AF motor works fine and can focus on the target if I use the live view (LCD).
I tried the single point auto-focus, Reset to Factory Setting, and I tried cleaning the contacts but still cannot get the AF to focus on the target through the viewfinder.
What could be wrong and what can I do about it?