This specialized ring is designed specifically for use with the Zeiss C/Y Contax 18mm f/4 prime lens. It attaches to the front of the lens via three small set screws. The outside diameter is a clean 80mm surface for mounting clamp-on accessories to. It also provides a 77mm filter thread for screw-on filters. Vignetting may occur with certain large filters on 35mm full-frame formats. No vignetting on Super 35mm format. Includes three set screws and hex-wrench required for installation.