[Leica] Re: Using the Russar 20mm on the Bessa R or L and other odd ball combinations

[Cummer Family <cummer@asiaonline.net>]

Dear LUGGERS,
From the archives May 31, 2000 Doug Richardson tells all:
Howard.
>If you use a "Voigtlander" Bessa or Konica Hexar, and fit only the
>company’s own lenses or modern Leica lenses, you may cheerfully ignore
>this posting. However, if you like experimenting with older Leitz and
>non-Leitz lenses on your camera, read on.
>
>It’s tempting to assume that any lens which will fit a Leica will also
>fit the Japanese-built Bessa and Hexar cameras, but this it not always
>the case.
>
>A few days ago Howard Cummer was passing through London, and wanted to
>try some of my older Leitz and non-Leitz screw-mount lenses on his
>rangefinder Bessa.
>
>Howard was interested in trying my KMZ 20mm f5.6 Russar, but found
>that the protruding rear section if this lens fouled on the internal
>structure of the Bessa, preventing it from being screwed home.
>
>The was a simple problem whose after-effects can be removed by the
>application of a little matt black paint to the metalwork which had
>made inadvertent contact, but a more complex problem emerged when
>Howard tried my 5.8cm f1.5 Leica-Sonnar. (This is a lens which the
>average Bessa or Hexar user is very unlikely to meet, but it’s
>possible that other oddball lenses might have the same problem.)
>
>The lens would not unscrew from his camera, but would turn only a few
>degrees anticlockwise before sticking!
>
>Reasoning that the problem might be due to the rangefinder cam, I
>focussed the lens to its shortest distance, and was then able to
>unscrew it. Looking at the rear of the lens, it soon become clear what
>the problem was.
>
>While a traditional Leica lens has a cam whose rear face lies parallel
>to the rear if the mount, and which moves in and out to couple to the
>rangefinder, the cam on the Leica-Sonnar and the 40mm Summicron-C for
>the CL Summicron-C (and presumably the 90mm C lens) has a sloped
>rangefinder cam which  achieves its effect by a combination of moving
>in and out and by turning to bring a higher or lower section of the
>cam against the rangefinder sensing roller.
>
>When a lens set to infinity, the cam is as far rearward (i.e. deep
>into the camera body) as it will go, and the position of the
>rangefinder roller is thus as deep into the body as it needs to go. As
>the lens is refocussed to shorter distances, the cam retracts into the
>lens, and the rangefinder roller moves forward
>
>On the 40mm Summicron-C, turning the lens mount from infinity to
>closer range not only retracts the cam into the body of the lens, but
>also brings a lower point of the cam into contact with the rangefinder
>roller. The slope of the cam thus increases the effect of the cam
>retraction. On the 5.8cm Leica-Sonnar the slope works in opposition to
>the cam retraction. Turning the lens mount from infinity to closer
>range retracts the cam into the body of the lens, but brings a higher
>point of the cam into contact with the rangefinder roller.
>
>The problem with the latter scheme is that when the lens is set to
>infinity (and the rangefinder roller is as far back into the camera
>body as it needs to go), other parts of the cam penetrate even deeper
>into the body. So if a lens of this type is to be screwed into and out
>of the camera body while set to infinity, the rangefinder roller must
>be able to move a further 2mm into the body in order to ride up and
>over the highest point of the cam.
>
>On a screw-mount or M Leica, the rangefinder mechanism can cope with
>this extra movement, but it seems that the Bessa rangefinder will not.
>
>Like many screw-mount enthusiasts, Howard sets a lens to its shortest
>distance before fitting it to the camera. Before trying to remove the
>lens, he’d refocussed it to infinity, and the rangefinder roller had
>been unable to cope with the highest point of the cam, so prevented
>the lens being turned further.
>
>At this point Konica Hexar users are probably saying to themselves,
>"My camera has a bayonet mount, so I’d have no problem, even if it
>turns out that the Hexar rangefinder roller cannot move deeper than
>the infinity position."
>
>Care may still be needed however. All Leica lenses (and most
>Leica-compatible lenses from other manufacturers) turn anticlockwise
>as seen from the front to change the focus from infinity to closer
>distances. However, the Leica-Sonnar (and perhaps some other non-Leitz
>lenses) turns in the opposite direction!
>
>On any variable-height (sloped) cam lens there is obviously at some
>point around the circumference of the cam when the low part meets the
>high part. With the Leica-Sonnar fitted and focussed to infinity, the
>rangefinder roller is sitting at the lowest point on the cam, and just
>to its right (as seen from the front of the camera) the cam slopes
>upwards at an angle of around 30 - 40 degrees to reach its highest
>part. The act of mounting and unmounting the lens will thus bring this
>high point against the roller, requiring the latter to move the extra
>2mm deeper into the camera body, which it may or may not be able to
>do.
>
>So if you want to try another manufacturer’s lens on your Bessa or
>Hexar, I’d recommend that you make sure that the lens uses a
>Leitz-style parallel cam. If it does not, check where the step in the
>cam lies when the lens is focussed at infinity and minimum distance,
>and how that step will move as the lens is mounted and removed. The
>safest measure will probably be to mount and unmount the lens with its
>focusing set to minimum distance to draw the cam as far as possible
>into the body of the lens. (The most important exception to that rule
>is the dual-range Summicron, which has two cams at different heights,
>and which should be fitted and removed with its focusing mount set to
>infinity.)
>
>Regards,
>
>Doug Richardson
*Thanks Doug.
* Howard