Nikon digital glasses have a small circle on them.

1, which should be the spiral fixing ring for fixing the front lens. It is used for fine adjustment of focal length, so it is also called focus ring.

2. There are problems in the process of lens processing, and the force of clamping the lens is too large, which leads to the phenomenon that the film in the central area of the lens blows up a little. If you have just got a new mirror, you can go to the optical shop and ask for a new lens for free.