The Sea & Sea 58121 is a set-up ring to attach the Close-UP Lens 125 to the DX-1200HD housing. Sea & Sea DX-1200HD Lens Adaptor for Close-Up Lens Construction: Durable Corrosion Resistant Aluminum Alloy Compatibility: Sea & Sea DX-1200HD Housing Purpose: Attach Close-Up Lens to Sea & Sea DX-1200HD Housing Attachment Type: Threaded