I always disassemble FD SSC lenses and machine down the stock. Once I determined the reduction amount it was the same for five different SSC lenses (50 1.4, 24 2.8, 28 2.0, 85 1.8, 35 2.0) I converted. Infinity tuning required only a minute tweak.
Machining provided several advantages. Obviously the lens group is set safely aside from the hazards of the workshop but also the stock can be properly positioned in the lathe to guarantee accurate material removal. Lapping and less precise methods could result in irregularities that could cause the lens to be mounted such that rear element is closer to the camera sensor on one side as compared to the opposite side.
When you are working within fractions of a millimeter for the lens to be mounted at the correct register distance it is quite possible that one part of the rear element could be at the correct register distance while the opposite side could be out 1/4 of a millimeter or more without the difference at the mount being detectable with the naked eye.
The disassembly is easy and safe. Take pictures across the rear of the lens with the lens positioned at infinity before disassembly to get a ballpark position of the rear element relative to the stock. Also use a marker to mark the position of the lens group relative to the stock. When disassembling the lens group from the helicoid keep your marker nearby. Slowly rotate the lens group out of the helicoid. At the exact point of separation, hold the two parts in one hand and make a mark on the group and helicoid. When you reassemble the lens you simply line up these marks as the starting point and gently \"work\" the lens back into the threads of the helicoid. If you start the group in \"too early\" the marks you made at the opposite end of the group will never line up. If you start \"too late\" you will pass the mark and have to \"wind back\" to get them to line up. Once you have done it a few times and get used to positioning the parts properly to each other to start threading and also marking things properly, everything will go back together properly on the first attempt.
I realize that not everyone has access to a lathe as I do. I just thought it was important to point out the necessity to maintain the highest level of accuracy within the limits of the tools that one uses to carry out this kind of modification.
Aug 25, 2009 at 03:05 PM
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