1-Cam, 2-Cam and 3-Cam R-Mount Lenses: A Complete Explanation

When photographers new to the Leica R-mount system first encounter the terms "1-cam," "2-cam," and "3-cam," the immediate reaction is often confusion. Unlike most SLR systems, which communicate aperture electronically and invisibly, the R-mount uses a physical mechanical system of precision-machined cams on the rear of the lens barrel to transmit aperture information to the camera's meter. Each cam serves a different metering system, and lenses were produced with one, two, or all three of these cams depending on when they were manufactured and for which generation of R-mount body they were originally designed.

This article explains the system completely: what each cam does, which lenses have which cams, how to use any cam-count lens on any R-mount body, and what the cam count means when adapting R lenses to modern mirrorless cameras.

What a Cam Actually Is

At the rear of every Leica R-mount lens, visible when you remove it from the camera, there is a circular ring that rotates when the aperture ring is turned. On this ring sit one, two, or three small mechanical followers — the metering cams — that protrude at different positions around the ring's circumference and move against corresponding followers inside the camera body as the aperture changes. Each cam profile is precisely shaped to move its corresponding follower by a specific amount for each increment of aperture change, effectively communicating "the aperture is now f/2.8" or "the aperture is now f/8" mechanically rather than electronically.

The three cam positions correspond to three successive generations of R-mount metering system, each introduced with a new camera body that required its own cam geometry. A lens with all three cams — a 3-cam lens — is fully compatible with every R-mount camera ever made. A lens with only one cam — a 1-cam lens — can only operate with full open-aperture metering on the original Leicaflex Standard; on every other R body, it requires stop-down metering.

The Three Cams and Their Corresponding Bodies

Cam 1 — Leicaflex Standard and SL metering. The original cam, introduced with the Leicaflex Standard in 1964 and used through the first generation of Leicaflex optics. This cam supports the stop-down metering of the original Standard and the open-aperture secondary-mirror metering of the Leicaflex SL introduced in 1968. Lenses carrying only this cam are "1-cam" lenses.

Cam 2 — Leicaflex SL2 open-aperture metering. Introduced with the Leicaflex SL2 in 1974, this second cam enabled full open-aperture metering on the SL2's improved through-the-mirror metering system. Lenses with both the first and second cams are "2-cam" lenses, and they support full open-aperture metering on the SL, SL2, and original Leicaflex.

Cam 3 — Leica R3 and later open-aperture metering. Introduced with the Leica R3 in 1976, the third cam enabled full open-aperture metering on the new electronic bodies from the R3 onward. Lenses carrying all three cams are "3-cam" lenses and provide full open-aperture metering on every R-mount body in the system's history.

The Compatibility Matrix

The following shows which metering modes are available for each cam count on each body generation:

How to Identify Cam Count on a Lens

The quickest way to determine a lens's cam count is to remove it from the camera and look at the rear bayonet ring. The metering cams are the small, protruding metal followers arranged around the ring. Count them: one, two, or three. Simple.

A secondary method is to check for a small engraved or stamped number near the mount — some lenses have had their cam count engraved as "3" after a conversion, though this is not universal and the physical count is always more reliable.

As a general guide, lenses with original production dates before approximately 1968 are typically 1-cam; lenses produced between 1968 and 1976 are typically 2-cam; lenses produced from 1976 onward are typically 3-cam. However, many lenses were updated mid-production, and many were converted by Leica service centres after purchase, so dating alone is not reliable. Count the cams.

Converting Lenses to 3-Cam

Any 1-cam or 2-cam R-mount lens can be converted to 3-cam specification by Leica Camera AG or a qualified independent Leica service technician. The conversion involves modifying or replacing the aperture coupling ring to add the missing cam(s) to the correct geometric specification.

Leica offered this service formally for many years, and many lenses in circulation have already been converted. A converted lens typically has a small "3" engraving near the bayonet, though as noted above this is not consistent across all converted examples.

The cost of conversion varies by technician and has historically been in the range of €80–180 depending on the lens. For a lens you intend to use frequently on an R3–R9 body, this conversion is almost always worthwhile — the workflow benefit of open-aperture metering over stop-down metering is significant in regular shooting.

For a lens being used exclusively on mirrorless cameras via adapter, cam conversion is entirely unnecessary and the cost is wasted — as explained below.

Cam Counts and Mirrorless Adaptation

When R-mount lenses are adapted to mirrorless cameras — Sony E, Nikon Z, Leica SL, L-mount — via passive mechanical adapters, the cam count is completely irrelevant. There is no mechanical metering coupler in the mirrorless body for the cams to engage; the camera meters through the sensor directly, reading whatever light the lens transmits at the set aperture. A 1-cam lens from 1965 and a 3-cam lens from 1995 behave identically in this context.

This has a practical implication for budget-conscious buyers: 1-cam and 2-cam lenses are typically significantly cheaper than their 3-cam equivalents on the used market, because the stop-down metering limitation makes them less desirable for use on R film bodies. For a photographer who will use R-mount glass exclusively on a modern mirrorless camera, these older, cheaper lenses offer identical optical performance at meaningfully lower prices. The saving on a 1-cam Summicron-R 50mm over a 3-cam example can easily amount to €100–200 for the same optical quality.

Which Cam Count Should You Buy?

The answer depends on your intended use:

• Using on Leicaflex SL or SL2: Any cam count is fine. Both the SL and SL2 provide full open-aperture metering with 1-cam, 2-cam, and 3-cam lenses alike. There is no reason to pay a premium for 3-cam if your body is an SL or SL2.
• Using on R3 through R9: Buy 3-cam lenses for the most convenient shooting workflow. Stop-down metering with 1-cam or 2-cam lenses is workable but adds steps to every exposure. If budget is the primary concern, a 1-cam or 2-cam lens can be used effectively with stop-down metering, or converted to 3-cam by a service technician.
• Using on mirrorless cameras only: Buy whichever cam count gives you the best combination of optical condition and price. Cam count has no effect whatsoever on the image when adapting to mirrorless.
• Building a versatile kit for both film and mirrorless: Buy 3-cam lenses. The premium over 1-cam and 2-cam examples is modest, and you gain the flexibility of using the same lens on any body in the system without compromising metering workflow.

The R-Only Cam: A Note on Leicaflex-Era Bodies

There is one additional wrinkle worth noting for photographers using the original Leicaflex Standard (the 1964 body, preceding the SL). This camera uses only stop-down metering, regardless of cam count — its meter physically cannot read aperture data from any cam. On the Standard, all R-mount lenses require the photographer to stop down to the shooting aperture before reading the meter, then reopen if using a fast lens for focusing. This is the same workflow as any stop-down metering camera of the era, and experienced photographers find it natural with practice; it is simply worth knowing before you pick up a Standard expecting open-aperture convenience.

Practical Impact of Stop-Down Metering

For photographers who have only ever used cameras with open-aperture metering, the stop-down workflow with 1-cam or 2-cam lenses on R3–R9 bodies deserves a realistic assessment. It is slower than open-aperture metering. It requires an extra physical action before reading the meter. And it dims the viewfinder — sometimes significantly at f/8 or f/11 — which makes composition and focus assessment harder.

That said, many photographers who use the Leicaflex Standard or who regularly shoot with 1-cam lenses on later bodies report that the workflow becomes entirely automatic after a period of practice. The extra step takes perhaps two seconds once it is habitual. For contemplative photography — landscapes, still life, architecture, portraiture with a cooperative subject — this is not a meaningful limitation. For fast-moving documentary or street photography where you need to react instantly to unfolding situations, open-aperture metering is genuinely more practical, and a 3-cam lens on a compatible body removes a point of friction from the shooting process.

Ultimately, the R-mount's cam system is a historical quirk of the system's three-decade evolution rather than a fundamental design flaw. Every combination of body and lens works; it is merely a question of how conveniently it works. Understanding the system fully allows you to make deliberate choices rather than discover limitations by accident — which is, given the cost of film and the time spent shooting, the more satisfying path.