Reverse Ring for attaching a 52mm diameter lens on the case, but in reverse, thereby making the macro. The ring is metal, not "cheap" so. Warning, this ring does not put a 52 mm lens mounted upside down on another goal 52 mm. This ring is only used to connect to a goal box. You can not use a lens focal length of 18-55mm and another 50mm focal mounted upside down on the first. Moreover, the use of the ring with a 18-55 is not recommended: in 55mm, zoom prevents the entry of light, and it is very difficult to obtain a sufficiently lit shot. Prefer the use of this fixed ring with a 50mm focal length lens. The photographs will then be up to your expectations!
