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Leica R-System Mount

The Leica R-mount was introduced with the Leicaflex in 1964 to enable photographers to quickly change lenses on Leica's then new line of 35mm single-lens reflex cameras. It is a bayonet mount with three tabs around the base of the lens that fit into corresponding recesses in the camera's female part of the mount. The flange-focal distance amounts to 47mm. Most lenses with an R-bayonet can be used on any R-system camera body, but not all camera functions might be supported, as the mount has undergone several modifications over time.

Three cam lens
The location of the three cams[coocan.jp]

In particular, five different types of R-system lenses exist that differ in the way that they transfer aperture settings between the lens and the camera. This information transmission is achieved through a cam that is linked to the movement of the aperture control ring. The cam is in contact with another mechanism in the camera that is coupled with the meter, which makes it possible to mechanically transmit information on the selected aperture from the lens to the camera. Leica has over time switched from chrome-colored, curved cams (in German: Steuerkurven) to black, three-stepped cams (Steuernocken) supplemented by a ROM chip.

Single-cam lenses (1964-68): Lenses that were released in conjunction with the original Leicaflex had one curved chrome-bar between the mount and the rear lens element to transfer the aperture setting on the lens to the camera. These single-cam lenses (alternative called "one-cam" or "1-cam"-lenses) can be physically mounted on later Leicaflex SL and R-cameras, but the cam does not transmit the aperture setting to the newer cameras, so that only stop-down metering is possible. In other words, the aperture has to be closed down to the intended f-stop for the camera metering to work properly. However, Leica recommends not to use single-cam lenses on the R8 or R9, as heavy usage of the old lenses in the past might have led to a slight misalignment of the cam that could damage electrical contacts in the female part of the mount of the latest SLR cameras.

3-cam lens
Detail view of the second and third cam[leica-wiki.de]
Twin-cam lenses (1968-76): With the introduction of the Leicaflex SL that supported TTL-metering, Leica changed the lens-to-camera coupling mechanism and moved the curved cam to the opposite side of the lens mount. To nevertheless ensure that the newer lenses could also be used on the original Leicaflex, the first cam was maintained, resulting in twin-cam lenses (alternatively called "two-cam", or "2-cam" lenses). The first cam does not have any function on the Leicaflex SL, but is used to set the viewfinder display of the aperture in the Leicaflex SL2. Similar to single-cam lenses, twin-cam optics can also be used on R-cameras using stop-down metering, but are not recommended in conjunction with the R8 and R9.

Triple-cam lenses (1976-96): The R3, which was developed in cooperation with Minolta, introduced once again a different coupling mechanism. This third cam takes the form of a triple-stepped, black projection at the inside of the second cam. The first and second curved cams were maintained to ensure backward compatibility of the lenses. The resulting "triple-cam"-lenses (alternatively called "three-cam", or "3-cam" lenses) function on any Leica reflex camera in open-aperture mode, and can be used with the R8 and R9.

Rom lens
Leica ROM lens-mount with R-cam[kehblog.com]
R-cam lenses (1986-09): Ten years after the Leicaflex SL2 was discontinued, Leica started to produce R-lenses that only had the third, stepped cam, but lacked the first and second cams. These lenses would, hence, not transmit any aperture information to the Leicaflex/SL/SL2 models, and Leica indeed changed the shape of the lens mount slightly, so that these R-cam lenses (alternatively called "R-cam only", "R-only", "three-cam only", or "3-cam only" lenses) could not be mounted on the Leicaflex/SL/SL2.

ROM-lenses (1996-09): The R8 featured electronics that could take advantage of lens-specific information to correct for lens vignetting, to adjust the zoom reflector on flash guns according to lens focal length or to correctly display aperture information if accessories, like tele-extenders, were attached to the lens. This lens-specific information was stored in a Read-Only Memory (ROM) chip that was added in the form of a strip of electrical contacts between the mount and the rear element of R-lenses. The ROM chip came with all newly sold lenses at the time, but could also be retrofitted by Leica technicians to older lenses. ROM-lenses have the R-cam and can also be mounted on R3-R7 cameras, where they support all the functions that these cameras offer.

Hence, it is worthwhile to carefully check the compatibility of R-system lenses and cameras.

Compatibility of Leica R-lenses
 LeicaflexSL-SL2R3-R7R8-R9
Single-cam+oo(o)
Twin-cam++o(o)
Triple-cam++++
R-cam--++
ROM--+++
Notes: "++": transmission of aperture and lens information; "+": transmission of aperture information; "o": stop-down metering only; "(o)": not recommended; "-": can not be mounted.

Some lenses with a long production history have appeared in several cam-versions, so that it is important to ensure that one's camera and lens are compatible when making a purchase in the used market. That said, it is possible to have the cams of most (but not all) lenses upgraded in order to make an older lens fully compatible with newer cameras (or vice-versa). For example, a single-cam lens can be upgraded to triple-cam and an R-cam lens can be converted to triple-cam. According to Leica expert Douglas Herr, this service costs between USD 140 and USD 200 per lens. However, as the ROM chip occupies the same place in the lens mount as the first cam, it is not possible to use ROM-lenses on the Leicaflex Standard.

Further Reading

On separate pages, you will find comprehensive compendiums with the key specifications of all the R-system cameras and all the R-system lenses. Moreover, if you need further detail, you might want to check out the selected Leica brochures that are available for free download, or consult your R-camera's manual. Last, but not least, the Leica family tree provides a visual impression of the evolution of Leica's cameras over time.