Hasselblad Medium Format Extension Tube 21mm (40010).

Number 21 refers to the length of the tube in millimetres (21mm). Can be used individually, or in combination with other tubes. Not recommended to combine more than three tubes.Must be attached to the camera body before attaching the lens to the tube. When using more than one tube, attach the first one to the camera body and then build out, attaching the lens last. Conversely, remove the lens from the extension tube first before removing the extension tube from the camera body. If using more than one tube, remove the tube closest to the lens first and work back toward the camera body.