Archived posting to the Leica Users Group, 2001/01/12

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Subject: R-cams, more than you ever wanted to know (was:Re: [Leica] Re:50 Summicron-R)
From: Doug Herr <>
Date: 12 Jan 2001 07:24:06 -0800

On Fri, 12 January 2001, "Randy Andrade" wrote:

> Please tell me if I've got it straight.  There are the two "normal" cams
> that look like what one would think of when one hears the word cam...little
> slopey thingies, for brevity's sake.  Then there's the aperture control or
> diaphragm lever.  But the third cam is the stepped "out" section of the
> metal that surrounds the rear lens element. This arc of metal runs about 1/4
> of the circumference, located inboard or radially closer to the center or
> axis of the lens from the lower or second silver cam, (usually at about 3 to
> 6 o'clock when looking at the lens from the rear in the normally installed
> and locked position).  Is this correct?  Does this cam move something inside
> the camera body, or is it supposed to move something within the lens itself?
> Thanks for all the help!
> Randy

The 3rd cam isn't as long as you've described it.  Yes, it's located inboard or radially closer to the center or  axis of the lens but it's not any longer than the 2nd cam.  With the lens off the camera body, move the aperture control ring and you should see all 3 cams moving in synchrony (as well as the aperture control lever).  If the thing you're looking at doesn't move with the slopey cams, then it's not the 3rd cam.

All the cams do is tell the camera body's meter what aperture the lens will be at when it's stopped down to working aperture.  In the case of the 1st cam, it tells the camera what the actual aperture is (for the Leicaflex Standard's external meter).  The 2nd and 3rd cams tell the camera what the working aperture is relative to full aperture, i.e., how many stops the working aperture is smaller than full aperture.  This is what the TTL meters of Leicaflex SL and all subsequent R-cameras need to know.

It may seem silly to have both the 2nd and 3rd cams doing the same thing but it can be explained with a review of the R-cameras' history: when the Leicaflex was developed, Leitz used the same technology for transferring aperture info (the 1st cam) as they had used for transferring focus distance to the LTM and M cameras.  The SL's 2nd cam merely followed in this tradition, and likewise, the SL2 used both the 1st and 2nd cams, the 1st for its viewfinder aperture display, and the 2nd cam for metering.  The SL2 was built in the mid-1970s when Leitz was in financial trouble, the SL2, seen by the public as obsolescent technology, was losing money for the company, and the company had little funds for R&D.  A few years before, Leitz and Minolta entered a technology-sharing agreement, and some of the earliest results of the technology sharing appeared in the SL2 in the form of a re-pivoted mirror which allowed lenses with Minolta's back-focus specifications (the distance between the rear element and the film plane) to be used.  Along with this change, some Minolta-built lenses became available in an SL2 mount: 16mm Fisheye, 24mm Elmarit-R, and the early 80-200 f/4.5 zoom (don't confuse this zoom with the current 80-200 f/4.0).

A further development of the Minolta agreement was the R3 camera body of 1976(?) which was derived from a Minolta body.  The Minolta camera used a meter coupling very different form the Leicaflex meter coupling.  I don't know if this forced Leitz to change to the R-style stepped cam, but the R-cam (a.k.a. 3rd cam) is much easier to work with than the Leicaflex cams when designing and using extension tubes and teleconverters.

The 3rd cam can be added to 1-cam or 2-cam lenses, and if you have any of the R-only lenses that have only the 3rd cam, usually the Leicaflex metering cams can be added.  ROM lenses replace the Leicaflex metering cams with electronic contacts.  These lenses cannot be used on Leicaflex bodies, but for a price many can be converted to 3-cam lenses.

Doug Herr
Birdman of Sacramento
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