I have been having the same F"0" problem with my D200 & 17-55 lens after taking long exposures (up to 30 seconds) and some regular shots intermittently. I switch off & on the camera and that fixes it temporarily. I have not cleaned the pins yet to see if that solves it. Seems to be a connection issue between the lens and camera. Most of the shots have been outdoors in relatively high humidity. The picture does take but turns out very dark (underexposed) when I pull it up in NX2.