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| p.7 #4 · lens repair, modification and disassembly guide thread |
OK, here's my version of the Contax G28 converted to Leica M mount, minus rangefinder focusing:
I originally, had a MD 50/2.0 because it was 49mm filter thread smaller diameter. I had a working solution except for the aperture, but the 50mm helical gave way too much focus travel. I also had problems machining as the helical was made out of plastic. So, it went on the shelf.
Then, on a whim, I bought a $40 MD 28/2.8. The focus helical and aperture matched up to the G28, so why not. The MD 28 also had a 49mm filter thread, and the helical turned out to be real aluminum.
The lens assembly sits back so deep into the Minolta 28 body that attaching a filter maybe problematic, but there really isn't any need for a lens hood. I infinity adjusted to maximize corner performance on the NEX-7, which means wide open hard stop infinity is a hair short of infinity. I typically don't shoot infinity scenics wide open, but at least f/4. I don't remember the G28 as having any field curvature, but there could be that symmetrical lens/corner sensor filter problem manifesting itself as field curvature.
The G28 minimum focus distance correlates well with the distance scale on the Minolta lens. Because the aperture movement was about the same radial distance between the G28 and Minolta aperture ring, I was hoping for some correlation there, but nope. That's why most lenses have the aperture cam, the original in the middle, replaced with a new piece that will operate the G28 aperture.
The aluminum helical was machined to give more room on the bottom for the aperture finger to reach up to the G28, and the top was surfaced in order to inlay the G28 optical unit into the helical.
A quick check of aperture ring accuracy showed a doubling of exposure time with the move from f/2.8 to f/5.6 on the Minolta aperture ring.
The couple of caveats aside, the converted lens is M mount for widest application, the focus and aperture rings turn in the right direction, no lens hood is needed, great Minolta mechanics and the package turned out to be reasonable in cost.