Nikon D3100 Lens Mounting Ring Reseating/Replacement
Introduction
If you can see black rings around the outside of the image preview, the lens may not be mounted correctly or the lens mounting ring has worn down and no longer holds the lens in position. If you have this problem use this guide to reseat or replace the lens mounting ring.
Reseat or replace the lens mounting ring and reassemble.
The locating pin on the right of the camera will help place the mounting ring properly.
The small metal plates marked in the image are not fastened and may displace if the camera if tipped or bumped. Be sure these are properly in place before reassembly.