Scratch or otherwise mark the edges of the rings for alignment if you want the 50 mm to end up in any particular position once mounted.
File: 1D__0887.JPG File size: 496KB Image Serial Number: 000-0887 Camera Model: Canon EOS-1D Mark II N Camera serial number: 0000404701 Firmware: Firmware Version 1.0.0 Date/Time: 2005:12:14 21:20:21 Shutter speed: 1/50 sec Aperture: 4 Exposure mode: Manual Flash: Off Metering mode: Evaluative Drive mode: Single frame shooting ISO: 200 Lens: 17 to 40mm Focal length: 40mm AF mode: One-shot AF Image size: 1728 x 1152 Rotation: none Image quality: Fine White balance: Tungsten Color space: sRGB Custom Functions: CFn 2: No exposure without CF card CFn 5: Manual Tv/Av set: Quick dial/Main dial CFn 8: Top LCD panel: ISO, Back LCD panel: Remaining shots CFn 18: Hold Assist button for registered AF point CFn 20: AI Servo tracking: Slow User comments: "Scratch or otherwise mark the edges of the rings for alignment if you want the 50 mm to end up in any particular position once mounted."