Kenko KSDR-5249 camera lens adapter

52-49mm Step Down Ring Stepping rings, also known as adaptor rings, allow the use of filters and accessories that are slightly larger or smaller in size than the front screw thread of the lens.

Step-Down rings in particular provide a way to fit a smaller diameter filter or accessory onto a lens with a larger thread diameter. The Step-Down ring screws onto the lens's front thread and provides a smaller thread which can then be used for attaching other elements, including more Step-Down rings

Care must be taken using Step-Down rings in order to avoid vignetting.