If you have purchased an early Leica R-mount lens — a Leicaflex-era Elmarit-R, one of the original Super-Angulon-R wide angles, or any lens bearing the "Leitz Wetzlar" engraving from before approximately 1968 — you may have noticed that the metering on your camera behaves unexpectedly. On some bodies, the meter appears to give incorrect readings. On others, the meter simply does not respond to aperture changes in the way you expect.
This is the 1-cam problem, and understanding it is essential to using the full range of R-system glass correctly.
How Cam-Based Metering Works
The Leica R bayonet mount communicates lens information to the camera body via a system of precision-machined cams on the rear of the lens barrel. These cams rotate as the aperture ring is turned, actuating a mechanical follower inside the camera that tells the meter what aperture the lens is set to. This allows the meter to calculate correct exposures without the photographer having to manually enter the aperture value.
The number of cams on a lens determines which metering modes it supports:
| Cam Count | Era | Leicaflex (Stop-Down) | SL / SL2 (Open-Aperture) | R3–R9 (Open-Aperture) |
|---|---|---|---|---|
| 1 cam | 1964–1968 | ✓ Full metering | ⚠ Stop-down only | ⚠ Stop-down only |
| 2 cam | 1968–1976 | ✓ Full metering | ✓ Open-aperture | ⚠ Stop-down only |
| 3 cam | 1976–2009 | ✓ Full metering | ✓ Open-aperture | ✓ Open-aperture |
What Stop-Down Metering Actually Means in Practice
Stop-down metering requires a specific shooting workflow. Rather than setting your aperture and reading the meter with the lens wide open (which gives a bright, easy-to-focus viewfinder image), you must manually close the aperture to your shooting value before taking the meter reading. The viewfinder dims accordingly — sometimes significantly at small apertures — but the meter then reads the actual light level that will reach the film or sensor.
On the original Leicaflex Standard, stop-down metering was the only option for all lenses. There was a dedicated stop-down lever on the body for this purpose. On later bodies (SL, SL2, and all R-numbered cameras), the stop-down lever was retained but its use became optional for 2- and 3-cam lenses.
The Workflow for Stop-Down Metering
- Mount the lens and compose your shot with the aperture wide open for maximum viewfinder brightness and ease of focusing.
- Focus carefully at maximum aperture (depth of field is shallowest here, making focus errors easier to see).
- Set your aperture ring to the desired shooting aperture. The viewfinder will dim.
- Press (or slide) the stop-down lever on the camera body to engage stop-down metering. On many R bodies this is the small lever adjacent to the lens mount.
- Read the meter and adjust shutter speed (or aperture) until correct exposure is indicated.
- Release the stop-down lever and shoot. The aperture remains at your set value.
With practice, this workflow becomes natural and adds only a few seconds to the process of taking a picture. It is slower than open-aperture metering, but it is not imprecise — the meter is reading the actual light that will expose the film, making it in some respects more accurate than open-aperture methods that rely on the cam to communicate aperture values correctly.
Converting 1-Cam Lenses to 3-Cam
Leica Camera AG offered — and certain independent Leica service technicians still offer — conversion of 1-cam and 2-cam lenses to 3-cam configuration. The conversion adds the additional cam(s) to the aperture coupling ring, enabling full open-aperture metering on all R bodies.
The cost of conversion varies but is typically €80–180 depending on the lens and service centre. For lenses you intend to use regularly, conversion is worth considering; for collector pieces, originality may argue against it. If you are unsure, check the lens carefully — many lenses have already been converted by previous owners.
Using 1-Cam Lenses on Digital Mirrorless
When adapting R-mount lenses to mirrorless cameras, the cam count becomes irrelevant — there is no mechanical metering coupling through the adapter. All adapted R lenses, regardless of cam count, behave identically on mirrorless: the camera meters via its sensor, the aperture is set manually on the lens ring, and there is no stop-down requirement. The 1-cam "problem" disappears entirely in this context, making early Leitz lenses particularly appealing buys for mirrorless adapters: they are optically identical to their later versions and significantly cheaper on the used market.