... used between camera(or projector) and lenses ,it could mount kinds of canversion lenses ,such as telephoto lens ,wide ... angle lens ,fisheye lens,and so on.Adapter ... screws onto the front of any 25mm Lens, and allows you to convert to the 37mm threads ... of accessory items(such as filters,hoods ,flashes and lens converters)We have other rings 37-30 ...