Canon Digital Lens Mount for JK Optical Printer

Last crawled date: 4 years, 2 months ago

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.
Steps:
Mount the kit lens on a camera and manually set the aperture to full open. (I don't think this actually matters at all, but I'm superstitious.)
Take the lens mount off the back of the kit lens. Don't lose the 4 screws because we will re-use them later. You will see a green circuit board. Unplug the tiny cables that connect it to the lens body, but leave the cable that connects to the lens mount.
Print the .STL file (with support because there's a little pocket underneath). I used 3 "shells" and 40% infill to make sure the walls were thick and sturdy.
Line up the printed adapter with the Canon lens mount, so the protruding solder points on the mount fit into the matching pocket in the adapter. You may have some wiggle-room in the rotation dimension. The adapter has a little nub that protrudes. It will eventually line up with the white dot on the camera body. For now, it should be oriented between a screw hole and depression in the mount's flange.
Mark the holes for the 4 screws. (I didn't model the holes because I'm lazy and I'm not sure that all kit lenses have the same hole spacing - It's not symmetrical!)
Drill pilot holes and carefully screw the mount and adapter together. The screws will grab nicely into the plastic and should sit flush with the flange so they don't scrape against the camera.
All done, now put it together on your JK and enjoy proper Live View exposure preview! (The camera will report whatever aperture the old kit lens was set to before it was taken apart, but it won't affect your exposures.)
NOTE: The included files differ from the part shown in the photos. I went back and fixed a few things:
The nub that lines up with the white dot on the camera body was off a little bit. I moved it so it should fall exactly in the right place.
I added a bevel to the side of the ring, to make it easier to tighten the thumbscrew that connects the JK bellows to the adapter. (In the photos I just cut it down with a knife, but the current FreeCAD model and STL file include it.) If your JK thumbscrew is on the top or somewhere else, you can rotate the whole bellows by releasing the set-screws on the JK lens board and choosing a new orientation.
Notes:
Designed in FreeCAD 0.18
Printed in PLA at 0.2mm resolution on a Wanhao Duplicator i3