Re: [Leica] Rangefinder Cams (was Leica-Users List Digest V3 #114)

[Charles Babington <cbabing3@swbell.net>]

Charlie Trentelman wrote, in part:

> That "steeeper cut" [on the 40 Summicron for the CL] is really a different sort of cam -- on most M lenses turning the lens mount to focus moves a solid cam back and forth at an even rate-- accomplished by several sleeves of metal machined on threads together so focusing the lens moves the cam against the little focusing wheel at the same rate no matter which lens you use.<

I've seen this statement, about different kinds of cams, before
(probably on Gandy's site), but I've been a bit mystified because on
"most" of my M lenses the rangefinder wheel simply rides on the back of
the rotating focusing helical in the lens - as on the 40 Summicron.  The
only exceptions that I've seen personally are the longer lenses with
removable heads (my 90/2 Summicron and 135/2.8 Elmarit have the
non-rotating "cam", and I suspect the 135/4 Tele-Elmar I used to have
did too.)  But my 35 and 50 Summiluxes have rotating cams like the 40
Summicron, as did the 35/2 Summicron, 50/2.8 Elmar, 50 DR Summicron, and
50/3.5 LTM Elmar I've had in the past.  I don't remember for sure, but I
think both the fat and thin 90/2.8 Tele Elmarits that I've owned also
had rotating cams.  So in my experience, most lenses have the rotating
type of cam.  Is my experience too narrow, or do most of the other short
lenses use the non-rotating cam arrangement?  (I have no experience with
any of the lenses shorter than 35mm.)

The 40 Summicron focuses from infinity to ~2.6 feet in about 90 degrees
of focusing ring rotation, while the 50 Summilux takes about 180 degrees
of rotation to cover the same range.  So a degree of misalignment or
centering of the rangefinder wheel will make twice the difference for
the 40 Summicron as for the 50 (probably not an optically exact
statement, but you get the point).  On the other hand, if the
rangefinder wheel is properly aligned, the "steepness" of the lens cam
shouldn't make any difference.  (As best I can tell, the wheels on the
rangefinder arms are the same size on the M6 and CL, so they should roll
along the lens cams identically.)  And in my own experience, my 40
focuses just fine on my M6.

These are remarks of a tax lawyer, not an optical engineer, although I
have been looking at Leica rangefinder cams since the late 1950s.

Cheers,
Kip Babington