Leicas are expensive (ask me, I have three). But a Canon 7 does a lot of what a Leica M can do for an order of magnitude less price. However, what it can’t do is somewhat of a big deal – no shoe to mount accessory finders or a flash; and it’s not M mount so it can’t take my favourite lens, the Voigtlander Nokton Classic 35/1,4. I’ll tackle the latter first.
2. Preliminary measurements
The Leica M mount is larger in diameter than the L39 mount, and its flange-focal distance (FFD) is 1mm less at 27,80mm. So in order to convert the Canon 7 to M mount, we will need to take 1mm off of the FFD and bore out the lens throat. At a glance, this does not appear to pose many significant challenges, but the increased bore and depth of the M mount causes interferences with the:
• Slow speeds escapement, namely the pallet disengagement mechanism
• Closing (2nd) curtain pulley
• Light baffling in lens throat
• Chassis mounts of front body
These items will have to be removed, modified or moved to make space for the M mount.
3. Parts and tools
The Canon 7 was sourced from ebay as non-working, for something like $60. The second curtain was stuck open, which took maybe a half hour to fix.
For the lens flange, I opted to use one out of a chepo LM-NEX adapter. It’s cheap and saves me time and headache, but it’s also not as positive as a real Leica M flange. A real Leica M flange has a flat spring to pull the lens tight whereas this cheapo adapter just has cuts in the flange that are bent outwards to add to the thickness of the flange. It’s not great, but it works.
I went to the trouble of making a boring head to do the lens bore, and also used a rotary table; however those operations could have been just as easily done in a lathe with a 4-jaw chuck and some sort of fixture.
The Canon 7 uses non-standard (presumably metric, but very coarse) screws, so I got the smallest standard size screws I could find at the local fastener store (#2-56 NC), and a nice tap to go with them. A 4mm 90 degree carbide spot drill was bought as a countersink but it turns out to be just a touch smaller than the #2 head size (grr). I would have preferred to use metric fasteners but they aren’t as easy to find here in Canada.
To check the flange focal distance, I bought a surface plate and a 0-10mm indicator, and got a buddy to surface grind a riser block and 28,80mm gauge.
4. The build
I bought the Canon 7 on the 16th of February and completed this stage of the build on the 8th of November. I'd guess I have somewhere around 40 hours into the project; a lot of it spent scratching my head or disassembling and reassembling the camera to fix stuff that I put together incorrectly... haha.
1. Eyeballing it. It doesn't look impossible, so let's go for it.
2. Milling out the lens mount anchors on the chassis.
4. Indicating the M bayonet true in the 4-jaw (the backgear in this lathe was broken trying to remove the 3 jaw years ago… so 4 jaw in 3 jaw it is)
5. Taking material off of the M flange. The 1mm difference in flange-focal distance was reached by removing material on both the lens flange and the body.
6. Centering the lens bore (I’m using a NogaFlex indicator holder – highly recommended)…
7. Boring out the lens throat with the boring head
8. Setup for boring. The dial indicator is used to check the boring head offset.
9. Facing the body with an endmill and the rotary table. Not sure why I used a rotary table instead of the boring head.
10. Match-drilling the holes for the M bayonet. I’m using super glue to hold the M bayonet to the body, which can be broken off later on.
11. M bayonet holes countersunk (this is fast forward into the future). I only had a single flute 90 degree countersink at the time and just wanted to keep the project going, so I did it with a hand drill and the results are atrocious.
12. The closing curtain pulley has to be moved to clear a lug on the lens mount. I moved the pulley about 2,8mm aft and 0,7mm outwards. The intention was to move the pulley along the arc of the gear that it meshes with at the bottom, but the measuring and drilling was a bit eyeball and naturally this didn't work out in the end.
Note that the top and bottom of the pulley axle are not the same size (1,5 and 1,7mm, respectively)
13. Holes cut in light baffles to clear the lens flange. These were roughed with a Dremel tool and then finished with files.
14. The only other LTM-M conversion I've heard of is a IIIg, which after conversion didn't have slow speeds that worked. I was determined to have all speeds in the Canon 7, which wasn't trivial since the slow speeds escapement interferes with the M lens bayonet. This brass lever here disengages the slow speeds pallet for 1/15 and 1/30 - it had to be chopped and bent to clear the enlarged lens bore. The pallet disengaging lever on the escapement had to be removed as well.