I just recently purchased a Nikkor AF 50 mm lense (1.8f) for my D3200. I'm still very new to photography, so I started with the 1.8f. Anyway, I am unable to change to auto focus mode on my camera as the "M" is gray and can't be changed. Is there something I'm doing wrong? I can manually focus, but can not get to the option to change to AF on my camera, if that makes sense. Please advise. Thanks in advance!