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Why do R lenses come in 2/3-CAM or ROM mounts?

The Ascent of Cam

Everything depends upon the manufacture date of the lens in question…

Single cam

The original Leicaflex Standard camera, released in 1964, used a cam arrangement inside the lens mount to mechanically communicate the aperture information from each lens to the camera body (similar to the way the Nikon AI mount does it). As there was only one cam, ergo… "single cam".


When the Leicaflex SL, with its integrated TTL light meter, was introduced in 1968, a new lens cam was added for aperture indexing. To maintain backward compatibility with older Leicaflex cameras, new lenses were released with two cams, so they could aperture-index on both old and new Leicaflex bodies.


In 1976 Leica released the R3, the first of its R series SLR cameras designed in collaboration with Minolta. Because the new camera(s) had auto exposure features, this meant adding another cam, bringing the total to three. This third cam is a black stepped affair, located next to and just inside the chrome 2nd cam.


An added complication is that some newer R lenses are sold as 3rd cam only. These will only mount on R bodies and are easy to distinguish as they have a collar around the base of the stainless steel lens mount flange, to prevent you from mounting them onto older Leicaflex bodies.

If you want to know more info about the Leica R cam-scheme, Jem Kime has written an incredibly detailed description at:


ROM at the top

In the early 1990's, Leica decided to embed a ROM chip into each lens, the idea being that it would communicate electronically with the camera body and tell it the exact aperture opening and focal length. Why bother? Noted R-user, Pascal Heyman, explains:

ROM does two things at this time: - steering of the motor zoom head on SCA 3501 compatible flash guns such as the Metz 45 MZ-3i or 32 MZ-3 (that is, the zoom head on the flash will adjust itself to the focal length of the lens used) - additional (to mechanical) electronic control of the light metering in P (program) and T (shutterspeed priority) mode, that is: the R8 will take several lens-typical factors in consideration when using program or shutter speed priority mode (such like: light fall-off in the case of wideangles, any kind of diaphragm malfunctioning and so on).
Additionally, and here I am quoting from information I received from the Leica information service in Solms: "The new data- and signal-transmission system between body and Leica R lenses are the bridge to the future of camera body and lens technology. They will allow further functions that make picture-taking easier, more effective and faster in the future."
As for the ROM with or without sensor, this has to do with the Vario lenses. The VE 28-70/3.5-4.5, VE 35-70/2.8 ASPH, VE 35-70/4.0 and VE 80-200/4.0 zooms have a sensor that tells the flash head the focal length position the zoom is in. However, this sensor is not present on the VE 105-280/4.2 and VE 70-180/2.8, where the flash head will always revert to the lens' minimum focal length. For example, if you turn the focal length ring on the 35-70/2.8 ASPH, the flash zoom head will switch from 35 to 50 and then to 70 automatically.
So, the moral of this is that you don't need the ROM contacts at the moment as they will only provide you with added but hardly essential benefits, but that you'd be better off with ROM contacts in view of future developments. This prospect is the reason why I personally don't want to buy R lenses without ROM contact strip.

Years later, ROM equipped lenses can also provide meaningful EXIF data for the DMR.

BTW the ROM chip has nothing to do with auto focus. There is some mention of this in the actual patent covering the ROM mount — see "Kunze US 4,786,934, issued Nov. 22, 1988 for a Photographic Camera Having An Interchangeable Lens, assigned to Ernst Leitz" — but nothing ever came of it.

To view the ROM patent online at the US Patent Office website, see their Patent look-up page and enter "4,786,934" in the query field.

For more detail about the ROM lens mount, see Leica Fotografie International, 7/98, pp.30-31

Which one to get?

You have no choice when buying new — all of Leica's R lenses are currently ROM + R-mount only.

But when buying 2nd hand, my advice would be to buy 3-cam lenses rather than ROM versions. The reason? Mainly because 3-cam lenses work on every Leica SLR body ever released. This will make your life much easier if you decide to re-sell your lens, as there are still thousands of Leicaflex SL(2) users out there.

In October 2005, Douglas Herr added:

[…] The flange is slightly different and a ROM lens can only be mounted on an R3 or newer body. The reason is that the ROM contacts occupy the space whee the first cam was. If a ROM lens were mounted on an SL2 or Leicaflex Standard, the ROM comtacts would be damaged by the body's cam follower. I recently upgraded my 80-200 ƒ4.0 from ROM to 3-cam (yes I consider this an upgrade). It now works perfectly on everything from the Leicaflex Standard to the R8.

Of course you can always have ROM lenses converted to 3-cam (see below), but why expose yourself to an additional expense if you can avoid it?

3-cam & ROM conversions

Getting ROM'ed

If you want to go from 2/3 cam to ROM then you have to have it done at the Leica factory in Solms. Not all lenses can be modified however - see the following Leica page for a list:


Back to 2/3 cam

You can have lenses backward converted from ROM to 3 cam by any experienced camera technician.

What does it typically cost? In May 2004 Douglas Herr noted:

[…] Leica USA charges $200 for the conversion. Don Goldberg (DAG) charges less and offers the option of leaving the first cam off, which means no metering with Leicaflex Standard and no aperture display in the viewfinder of the SL2. Without the first cam the SL and SL2 will still meter correctly. Don's conversion of my APO-R 280mm w/o the first cam was $140. Sherry Krauter also does this work and told me she'd charge less than Leica USA.

What lenses can be done? Douglas Herr again (June 2004):

Retrofitting a modern R lens to work on the SL or SL2 involves cams and the flange. There are small differences between the SL flange and SL2 flange and R flange. The differences are supposed to prevent damaging the lens or camera, either from the mirror hitting the rear element of the lens (certain combinations of the SL and specifc lenses) or from the follower for the first metering cam striking the R8 or R9 ROM contacts.
Some current lenses like the 180mm APO-Elmarit-R and the 100mm APO-Macro-Elmarit-R can be retrofitted to be fully compatible with the SL and SL2. Others, like the current 50mm Summilux-R require more mirror clearance and can be retrofitted to work on the SL2, but not on the SL.
My 280mm ƒ4.0 APO-Telyt-R was originally a 3rd-cam lens. Since it had an R-only bayonet it couldn't be used on the SL or SL2 but by modifying or replacing the flange and adding the first and second metering cams it can be made to be fully compatible with the Leicaflex SL and SL2.

A note about possible broken links

This FAQ has over 900 external links. Over time it is inevitable some of them will break. If you are bothered by this, see this detailed topic elsewhere in the FAQ.

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