Komura 200mm f4.5 rangefinder-coupled lens

[roydonian]

Advertisement (gone after registration)

Has anyone had experience in stripping down, relubricating, and reassembling the 200mm f4.5 Komura rangefinder-coupled lens?

 

The focus control of mine was very stiff (a common problem with this model of lens), so I had it relubed several years ago, but the technician who did the job may have failed to set the rangefinder cam properly. The rangefinder images of a distant object could not be made to merge.

 

The silver-coloured cover at the rear of the focussing mount unscrews, revealing a telescoping cylinder that has the rangefinding helical at its rear end. This cylinder turns as the lens is focussed.

 

The rear cover has two threaded sections. The outer and finer-pitched thread engages with a threaded section at the rear end of the focussing mount, while the central hole has a coarser-pitched thread that mates with that on the rangefinding helical. As the lens is focussed, the turning movement of the rangefinding helical alters the position of its rear edge, which actuates the rangefinder mechanism in the camera.

 

Since the rangefinder helical and the threaded section of the rear cover into which it fits both have a simple thread with only one entry point, the reassembly task should have been a fairly simple.  Getting the coupling correct seems to be a matter of making sure that as you refit the rear cover, its inner thread must touch then engage the rangefinding helix at just the right point to give the required rearward protrusion when the cover is screwed fully home and the lens to set to infinity.

 

Screwing down the rear cover when the lens is set to infinity results in the cam protruding too far, so the obvious procedure was to set the lens' focus control to a position away from infinity, in order to move the rangefinding helix away from the rear of the lens, delaying the point at which it will engage the screw thread of the rear cover as the latter is screwed into place.

 

As I anticipated, a process of trial and error has produced positions in which the cam either protrudes too far or two little, and one that at first sight seems right. But the latter still produces the same sort of small rangefinder error that the lens had after the relube job. I find it hard to believe that the lens was manufactured with a rangefinder error. Perhaps there is some factor that both I and the experienced technician who did the relube failed to understand as we reassembled the lens.

 

Someone on another forum who'd relubed his example a decade ago reported at the time that "Once I got the rf mechanism installed almost correctly (choosing the right thread entry) I managed to get both rf and optics correct and adjusted to each other by rotating the finer pitch thread and the coarser rf thread together. That way I managed to get them both right." But looking at the hardware, I can't work out what he meant.