More details about the mod:
1) I bought a cheap Topcor 53/2 lens. 50/2 should work as well, but I managed to screw up (pun intended) the helicoid of the lens I first bought (see posts above), so I had to shop for another one which turned out to be 53mm. The focus ring looks a bit cheaper in the 53mm though, so I would recommend a 50mm if you can find one.
Remove the optical assembly and all the aperture and bayonet locking mechanisms (springs, screws and levers). Remove both helicoids and mark their separation points.
Clean and set aside the helicoids for later.
2) I bought a cheap Fotasy M39-Nikon Z adapter. I removed the 2.5mm set screws that hold the M39 threaded insert and took out the insert. I then drilled and tapped the holes for M3 in order to use longer M3 set screws with pointed conical tip. After removing a hair of the inner diameter of the adapter with a rounded file, the Topcor rear shroud fits inside the Fotasy adapter. Position the two parts correctly, check for planarity with a micrometer, and then use a 2,5mm drillbit to make small conical holes in the Topcor rear shroud, aligned with the M3 holes in the Fotasy adapter. The M3 set screws will bite into those, holding the two parts together.
3) Remove the Topcor rear shroud. Use a Dremel cutoff wheel or similar to cut off the rearmost section of the shroud flush. It should look like this:
After putting it back into the Nikon Z mount, it will look like this:
Like this the rear shroud will not vignette. However, the shinyness of the cut surface is not ideal. At a later stage I will paint it black. Any suggestions on which paint to use?
4) Cut off the rear part of the Contax Zeiss 45/2 outer helicoid. Use a small file to remove a bit of material around the outer diameter. Here I did one pass:
Remove just as much as you need in order to be able to press it into the Topcor inner helicoid without excessive force (I used my hand and a few light taps with a mallet. Do not use more force as I suspect it might deform the helicoid (anyway, this is my only explanation for how I screwed up the previous one, where I used a vise to press in the part without having first removed a bit of material).
5) After re-greasing the topcor helicoids and reassembling them, the Zeiss inner helicoid can be screwed into the Topcor inner helicoid (with its new insert). Place the optical assembly:
Be sure to use a micrometer or caliper to check the planarity of the setup at each stage of assembly (both of two parts that are mounted together, as well as cumulatively, i.e. relative to the Nikon Z bayonet mount).
6) The whole rig can now be mounted onto the camera for infinity calibration.
Leave some leeway in the Topcor outer helicoid for later calibration by using the position of the focus ring before drilling some holes or using epoxy to permanently secure the Zeiss inner helicoid to the Topcor inner helicoid.
The front part of the lens is currently not finished. The 53mm lens focus ring has a 55mm thread, into which I can screw 55-58, 58-55, 55-52, and 52-55mm step-up/down rings. The Zeiss aperture ring sits in this stack and is being gripped by the rings. I can thus control the aperture. The main reason for putting this temporary solution is to prevent dirt from getting into the helicoid, which is otherwise exposed. The Zeiss front section (into which the front bezel is screwed) fits snugly in the 52mm ring, almost sealing off the whole lens. However, the aperture ring now rotates along with the focus ring, which is of course not very efficient in use. Finding a permanent and workable solution for the front will be the most challenging part, getting the lens to focus correctly was actually easier than expected!
To be continued...
