i brought a brand new d5300 50mm 1.8g and tamron 18-200 about 10 months ago. everything is fine. the camera and the 2 lens. i borrow a d300 for testing. i put 50mm to d300. everythings fine. it autofocus smoothly. i tested the front botton beside the cap. and all the sudden it doesnt autofocus. i put it back to my d5300 and also doesnt autofocus. i tried the tamron 18-200 to my d5300 and still doesnt focus. what the hell happened? please help me.
Interesting question. I bought the 50mm f1.8G a few months back. It went on and off my old D70 with no issues, but on my D810 it felt stiff when turning into the bayonet fitting. The lens release button also felt stiff, and autofocus didn't operate. When I took it off and tried another lens, there was still a problem with autofocus, but when I took that lens off, looked at the mount and contacts, and put it on again it operated as expected.
The cameras and lens seem to be fine ever since and I've also had that lens on a D3200, but now I've read your question I'm wondering whether the 50mm f1.8G mounts or electrical contacts are stiff when new. It's worth checking the electronic contacts on both the camera and the lens to see they're not misaligned or damaged - it looks like I got away with it.
Hope it works out for you.