Hi all, managed to pick up a Nikon 300mm f5.6 for my Nikon D7100 ... I've played around with my auto focus lenses without a hitch, but I cannot take a pic through the 300mm .. it is used so I have the chance to send it back ... but all I get is black screen .. any suggestions ? and no even though I am a complete novice .. I don't have the lens cover on 
