This is a solution to an obscure problem when using a Canon SLR with a JK Optical Printer (a device originally designed for optical effects and reframing of Super8 and 16mm movie film).
I sometimes use my JK with a Canon Rebel XSi to transfer Super8 and 16mm film to high-res digital files. To attach the JK bellows to the camera I used a modified Canon body cap, but I quickly ran into a serious problem when using Canon’s Live View in manual exposure mode: The Live View image is crazy dark even when ISO/Shutter/Aperture are dialed in to correct values. This makes it impossible to focus and frame, and causes exposure confusion since the brightness of the Live View doesn’t match the exposure of the frames being shot.
There are numerous threads about this issue on the interwebs. The source of the problem is a lack of electronic communication between the manual lens and the body, which confuses the Live View exposure preview algorithm. (If the body displays an aperture of “F00” you’re in trouble!) The smartest solution for most users of manual lenses is to buy an “AF Confirm chip” on eBay and glue it to the back of the lens mount. This fools the body into thinking there’s a digitally-controlled aperture and the exposure algorithm works correctly. But… I’m cheap and I happen to have a broken 18-55mm EF-S kit lens lying around, so I made an adapter that re-uses the mount from the kit lens. It solves the Live View exposure problem and provides a nice clean mount for the JK bellows.