Disassembly and repair - MS-Optics Sonnetar 73mm F1.5 Leica M mount lens

 This article is about one of most interesting lens I had chance to work with in last decade - MS-Optics Sonnetar 73mm F1.5 Leica M mount. Huge thanks to Fred Miranda for providing his brand new lens copy for experiments, and for extensive analytical collaboration on lens optical properties research.

MS-Optics lenses are very unique custom designed and produced in limited batches by Mr. Miyazaki in Japan. For more context here's short article about MS Optical Japan on Japan Camera Hunter , and great review of MS-Optics lenses on phillipreeve.net .

What is shockingly-impressive about MS-Optics - these lenses are extremely compact, I mean, unbelievably small and lightweight for their focal length and aperture combo! They are also very beautifully crafted and have unique image rendering too. And yes, lenses are made for Leica M mount with full support of rangefinder coupling, though remain at relatively affordable price level for M mount shooters.

Many assembly steps are performed manually including fine tuning of RF precision, which takes a lot of human effort. There is obviously a reasonable balance of mechanical quality level and lens total crafting costs. Some lenses copies may have minor flaws or imperfections that require further fine-tuning. The Sonnetar 73mm F1.5 lens I'm working on has two of them - Leica M camera is showing incorrect OVF framelines, and  rangefinder focusing is not precise.

Fixing OVF framelines

First issue is very easy to solve by masking mount area and minor grinding of M mount leaf responsible for triggering proper framelines in M mount cameras. Shaving less than 1mm of metal corrected framelines to be 50/75.

Fixing RF precision

Second issue is much harder though and may require dozens of effort hours to get properly corrected. Problem origin of current lens copy - is uneven surface of RF couple cylinder. Looks like minor number of Sonnetar 73mm F1.5 lenses have RF cylinder too narrow and extra material is added later to compensate cylinder width. It looks like a welding surface or dried epoxy, later grinded for RF calibration.

Unfortunately the edge finish is not even enough. You may compare it with temporary repaired road pavement surface with lots of bumps and holes. It's making Leica M camera RF couple roller to travel with jumping up/down instead of expected smooth roll, forcing OVF RF patch to also show incorrect focusing distance. As a result many pictures having back/front-focus leaving point of interest in defocused area, especially at F1.5 aperture.

Here are few more pictures showing uneven edge of RF couple cylinder (front and rear views).

I'm first masking thread areas, then applying thin layer of glue like material used for Gunpla modelling (thanks to my brother for gifting me one recently). Once it's dried overnight, I'm using metal file to fine grind RF cylinder edge to smooth and even surface, but leaving some material for further RF precision tuning with Leica M camera.

  Here's resulting RF couple cylinder after making surface flat. Now M camera RF couple roller is moving very smooth and delivers very precise OVF RF patch movement for accurate focusing.

Lens disassembly steps

Releasing of RF rangefinder couple cylinder requires partial unscrewing of three bolts of RF transmission, visible on the inner wall of lens mount. After that RF cylinder can be unscrewed out in CCW direction. 

Then I'd recommend to mark the detaching point of mount ring while rotating it CCW, and remove the ring.

You can see both inner wall threads in the mount ring very precisely machined - focus thread to the left and RF cylinder to the right.

Aperture chamber can be accessed any time without described pre-assembly by simply unscrewing front rear block at the edge of focusing ring.

Few more pictures of RF couple cylinder and transmission that is sliding inside rectangular groove of RF cylinder.

RF cylinder consists of two parts precisely glued together. First one is an anodized aluminum cylinder with outer thread. Second one is a gray circle made of some plastic, on most Sonnetar 73mm F1.5 lenses copies it is protruding higher above metal cylinder edge, and grinding it allows to fine tune RF precision right when mounted on lens. However in this particular copy plastic ring sits deeper, which explains why extra material was applied afterwards.

UPDATE: part two of the article

Fine tuning RF precision

It's important to remind that designing and producing lens from scratch is a huge challenge, especially if you need to keep lens affordable. So I personally think that all imperfections of Sonnetar copies and resulting quality variations are within acceptable range. It is still excellent lens out of the box and can produce precise focusing when using Live View, and close to good precision when shooting with film. Fine tuning lens mechanics precision requires additional labor hours, which would significantly increase each copy cost. With proper skills and guiding details and precautions it's also doable DIY project.

During continuous exploration of MS-Optics Sonnetar 73mmm F1.5 lens model, I had chance to work on two copies - first one provided by Fred, and second one with SN:125 purchased used from FredMiranda forum. That second Sonnetar copy is much earlier production batch and has few minor  cosmetic and mechanical differences:

  • gray plastic ring of RF couple cylinder has much higher edge elevation, allowing to grind it for RF precision
  • focusing distance scale is including "1m" indicator, and sign "Mtr." indicating meters
  • mount ring shell does not have DOF scale, instead there is engraved red dot
  • Leica OVF framelines are showing correct 50/75 without need of grinding mount leaf
Observing only two copies of Sonnetar does not provide representative enough statistical information, though this second copy also exposed few similar problems which first and newer copy had
  • short but noticeable lag of RF patch when changing focusing direction
  • "sticky" focus rotation at infinity point
  • RF couple cylinder is jumping up slightly when reaching infinity point
  • RF indication is not showing correct focusing, leading to front/back focused resulting images.
NOTE: thanks to the lens manual provided by Mr. Miyazaki, and additional Fred's detailed observations when experimenting with Spherical Aberrations settings of Sonnetar, I've set it to optimal setting "3" when working with further mechanical improvements and tuning RF precision.

I had to find a way to solve first three listed issues, otherwise precise RF couple ring geometry tuning would be significantly complicated.

Fixing RF lag when changing focus direction

The RF couple transmission ring has protruding rectangular pin, that is sliding inside corresponding vertical groove on the inner wall of outer threaded RF couple cylinder. It is transferring rotation of focusing helicoid to rotation of RF couple cylinder. Both copies had with of that rectangular pin of 3.5-4mm. The groove width is a bit wider, so when changing focusing direction, pin is moving first for some degree, and then turning RF cylinder. To eliminate that lag I've increased width to approximately 4.05-4.1mm by gently but controllably applying pliers pressure on flat pin surfaces.

Fixing "sticky" focus rotation at infinity point

RF transmission ring is connected to the edge of focusing helicoid cylinder and is retracting inside lens when focusing toward MFD. When lens is focusing toward infinity the RF transmission ring is extracting until it's movement is stopped by inner surface of RF couple cylinder gray plastic ring. At infinity point it is applying slight pressure at RF cylinder, and forcing it elevate a bit and produce light jamming of focusing ring effort leading to that "sticky" feeling of rotation.

Eliminating that issue requires two operations. First is to remove the old grease from RF helicoid and replacing it with more smooth grease, and I'm using Polar Bear Camera grease number 00.
Second step here is to loosen three bolts on RF transmission ring, then holding focus at infinity and rotating RF cylinder only few degrees CCW. That will form a very minor gap between RF transmission ring and gray plastic ring of RF cylinder. Of course as a side effect, it will slightly elevate edge of RF cylinder and increasing front focusing influence, but I'm planning to later grind RF cylinder edge anyway, so it's acceptable and necessary change. Then three bolts on RF transmission need to be tightened again.

At this point, focusing of both copies become very smooth at whole range including infinity point, and RF cylinder does not show any lags or shifts in the thread anymore.

Fixing RF patch focusing precision

Thanks to previous operations lens RF couple is showing significant front focusing at whole range. So next step is to accurately grind RF cylinder edge until it's lowered enough for each focusing distance to eliminate front focusing.

RF cylinder gray plastic ring can be grinded using metal file. Since it is not metal, it's relatively safe to perform it right ton assembled lens, which allows to frequently check focusing precision on Leica M camera equipped with Live View.
To reduce plastic dust migration I'm cutting paper tape circle and covering rear optical path. Then:
  •  accurately and gently grinding edge path of RF cylinder by up to dozen of gentle sliding moves of metal file at each focusing distance.
  • accurately wiping out dust, then blowing out remaining dust using air Rocket Blower
  • removing paper tape circle and mounting lens on Leica M10P camera
  • checking difference between RF patch and LiveView at distances: 1m, 1.2m, 1.5m, 2m, 3m, 5m, 10m, infinity.
  • detaching lens from camera, marking with pencil locations of focusing distance at RF cylinder I need to grind more
  • attaching paper tape circle back, then repeating these steps again
At the point when I notice that difference between RF patch and LiveView is reduced to very minor front-focusing, I'm stopping grinding RF cylinder edge for that focusing distance.
When all focusing range is showing only very minor front-focusing, I'm slowing down to just few grinding moves, and also making sure that RF edge is overall smooth without bumps of lowering - curvature should have similar shape over all range without rapid angle changes.

Finally after about 3-4 hours of accurate grinding and testing, RF edge of my Sonnetar copy is precisely tuned, and lens is now able to focus razor accurate at F1.5 aperture to any focusing distance.

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