Adapter ring is used between camera(or projector) and lenses,it could mount kinds of conversion lenses,such as telephot lens,wide angle lens,fisheye lens,and so on.
Adapter rings are labeled from the camera to the filter.
For example, a 25.5mm to 46mm step up ring is used for a camera (or lens) with 25.5mm threads. The adapter will allow a 46mm filter (or other adapter ring) to fit.
Adapter rings can be stacked. To use a 49mm filter on a 25.5mm camera lens, first use a 25.5mm to 46mm step up ring and then a 46mm to 49mm step up ring.