This lens conversion is one of my favourite personal projects, it was quite a bit harder than my previous lenses due to the addition of a rangefinder mechanism but this gave me a good understanding of the Leica focussing system and lens design in general.

As with all my conversions, this one started by taking the camera part and taking a few measurements to see if it was actually possible to mount on a Leica. Most of the time the camera in question ends up going to the spares drawer for future projects, but I got lucky with the electro. The lens has a focussing mechanism, aperture unit and will fit on a Leica, furthermore it is possible to add a rangefinder interfacing mechanism for focussing. 

The lens is removed by peeling back the leatherette and removing 4 large screws that hold the lens plate which the actual lens is mounted to. After taking the lens off I completely stripped it and removed all the shutter parts inside it. One regret was taking the aperture unit apart to clean it as took me a couple hours to get it back in together.

After lubricating and reassembling the lens I took some detailed measurements and started figuring out how to include the focussing mechanism. Leica's have a rangefinder cam inside the camera mount for focussing, it is a small wheel which is pushed by the lens as it is focussed. The lens must push the cam at a specific rate in order to show correct focus through the camera rangefinder. Leica designed this so that standard Leica 50mm lenses (Actually 51.6mm) are 'calibrated' to the focussing mechanism such that the cam is pushed at the same rate as the lens. Any lens that is even slightly off of 51.6mm will need a more complex mechanism which must rotate to compensate for the difference. (Think about how a 15mm lens only moves forwards a few mm to focus whilst a 200mm travels much further, these differences must be compensated for in the design of the focussing mechanism.

There's no useful information released on the Leica mount or focussing mechanism so I came up with my own based on measurements I took and tests I did on other Leica lenses. I did not actually figure out the problem with using lenses other than 51.6mm until half way through the project, luckily there was around 2mm extra to play with so I could just about squeeze a rotating (indexed) focussing mechanism in there to compensate for the ≈45mm focal length. Above is a CAD drawing of the mount and mechanism described, the two silver parts on the top are the mount (outer) and rangefinder interface ring (inner). The yellow part is a brass focussing helicoid which has the rangefinder ring pressed into it.

This part features the slot to hold the lens from rotating as it is focussed. Most of this part had to me machined away due to space limitations. I also had to take around 1mm off of some other small rings to get more space between the camera and lens.

Next I made up the most complex part, the mounting ring, which screws into the back of the lens and mounts to the camera. Here you see 2 because I messed up the first one by filing the frame line selector edge down too far, resulting in 35mm framelines being brought up. I redid the part, machining the mount bayonet, screw holes and mount locking slots. I also cut a relief to stop the part from hitting the rangefinder cam. Note the slots inside the ring, these fit into the part shown above when assembled.

Above is the assembly for the rangefinder coupling. The brass focussing ring has a small recess cut into it so the aluminum cam ring could be pressed into it. The aluminum ring was difficult to machine due to its low stiffness, there is only a few grams of material left on it at this stage.

Looking at the middle image, the slanted edge for the rangefinder cam can be seen at the front of the ring. There are also cutaways at the front and rear to allow other parts  to pass through. The slots and tabs shown in the mount parts shown previously have to actually pass through this ring which makes everything even more complicated. Again this is due to a lack of space, but this time in the radial direction.

The first part I machined fits to the back of the mains lens body (left). The brass focussing helicoid then screws around it. The mounting ring is then screwed to the outer helicoid ring and this is screwed to the outside of the brass focussing ring (right).

On the right is the completed assembly with everything apart from the outer focussing ring with the numbers. The previous images show the final assembly without the rangefinder ring, which has been added here in the right most image.

Here you see the final assembly. You can see just how close together the moving parts are in this, as well as the endless filing marks I made whilst getting everything to fit together, which was not easy. To calibrate the lens I bought a Fuji XPro-1 and adjusted the focussing ring to match that of a Leica mount Zeiss lens I had.

I am really happy with how accurate the rangefinder coupling came out in the end. I made the indexed edge by hand using a file so I'm surprised it ended up accurate throughout the whole range. I'll show some images I've shot with the lens below, thanks for reading!