Automatic Diaphragm Coupling
The diaphragm of a lens on a single lens reflex camera needs to be at its maximum aperture while focusing and picture composition are being done. This ensures that the screen image is as bright as possible and can therefore be visually assessed with ease. The actual photograph may need to be made with the lens set at an aperture smaller than maximum. Adjustment of the lens aperture is done mechanically with lenses fitted with an auto diaphragm.
The aperture of a lens fitted with an auto diaphragm remains at its maximum opening regardless of the F-stop setting on its aperture control ring - allowing the photographer to pre-select the aperture required for the picture, yet still view and focus comfortably.

Coupling lever
Immediately after the shutter release is tripped, but before the shutter operates, a mechanism inside the LEICA R or LEICAFLEX camera moves away from its contact with a coupling lever (see illustration) on the R lens. This allows the spring-loaded diaphragm to close down automatically to the pre-selected aperture.
The shutter then opens and closes to expose the film. The camera mechanism moves back to its original position and, in so doing, bears on the lens coupling lever to re-open the diaphragm to its maximum; the camera and lens are then ready for viewing and focusing the next picture. An auto diaphragm simplifies and speeds up camera operation.

Automatic diaphragm R Lenses

Key
1. Cam 1: Tapered shape with bright edge: For LEICAFLEX.
2. Cam 2: Tapered shape with bright edge: For SL and SL2.
3. Cam 3: Stepped fug with black finish: For LEICA R.
4. Red index for lens change.
5. Auto-diaphragm coupling.
Cam Versions
: Single Cam (Cam 1 only)
: Twin Cam (Cams 1 & 2)
: Triple Cam (Cams 1, 2, & 3)
: R Cam (Cam 3 only)

Aperture / meter feedback cams
Camera operation is simplified further if the exposure measurement can be made concurrent with viewing and focusing. But this means that the measurement will be made with the lens wide open, whereas the actual photograph may need to be made with a smaller aperture.
The meter in the camera must be 'instructed' by the tens as to its maximum opening and the relationship to it of the aperture to be used for the picture. This information feedback is achieved by a cam on the lens that is coupled with the movement of the aperture control ring. The cam is in contact with another mechanism in the camera that is linked to the meter.
As an example, consider the use of a f/2 lens that is set to f/8. The area of the f/2 diaphragm opening is 16x that of f/8. The exposure measurement made at f/2 is adjusted by 1: 1 6 by the action of the lens cam on the meter-linked camera mechanism.
Because the aperture / meter cams compensate TTL measurements as described above, stopped-down metering (using the Depth-of-Field Preview Control) should not be attempted. Otherwise, a double compensation is effected which leads to incorrect exposure, When the aperture ring is turned to smaller settings, the cam pushes the mechanism, when the ring is turned to larger openings, and the mechanism 'chases' the cam because it is spring-loaded. To ensure contact at all times. The mechanism is designed to over-travel (by its spring-loading), i.e. to be able to move a little beyond the furthermost position of any lens cam. (This characteristic will be referred to later when manual diaphragm lenses are discussed.)
Because of unavoidable differences in camera design, the nature of the meter-linked mechanism varies between some models. This means that lens cams of different types have been used and these are illustrated.
The principle adopted in manufacture is that lenses are made with the cam appropriate to the current camera, plus, wherever technically feasible, the cam(s) required by all discontinued cameras.

This has led to the following (general) situations:
1965 to 1968: Single Cam Lenses (1st Cam only)
1968 to 1974: Twin Cam Lenses (1st and 2nd Cams)
1974 to 1976: Twin Cam Lenses
1976 to 1980: Triple Cam Lenses, (1st, 2nd and 3rd Cams)
1980 onwards: Triple Cam Lenses
1986 onwards: Triple Cam Lenses, R Cam lenses 3rd cam only
Some lenses, for example those with a long production history, have appeared in Single Cam, Twin Cam and Triple Cam versions. Others have been made as Twin Cam and Triple Cam and R Cam
Second-hand lenses may be in their original Single or Twin Cam state or may have been converted to Triple Cam. The only reliable way of establishing their suitability for use on a given camera is visual examination. Absence of the appropriate cams means that the meter is not coupled and damaqe may be caused in some cases.

Conversions
: Single Cam to Triple Cam
: Twin Cam to Triple Cam
: R Cam to Triple Cam
For costs of conversions contact Service Dept.

UPDATE (around 1988?)
Since 1976 LEICA R lenses have been fitted with 3 cams to couple lens aperture settings to the meters in various cameras as follows:-
Cam 1 : LEICAFLEX (original) discontinued 1968
Cam 2: LEICAFLEX SL/SL2 discontinued 1974/76
Cam 3. LEICA R (all models)
The 3 cam arrangement (Triple Cam lenses) has meant that with few exceptions, lenses were compatible with all cameras. However the production capacity tied up in the manufacture and fitting of 3 precision cams to every lens made has caused the practice to be reviewed. As we have not made cameras requiring Cams 1 and 2 for 18 and 10 years respectively, we feel the need for these control elements has been met The following lenses are now made, therefore, with Cam 3 only:
11257 24mm F2.8 R
11247 28mm F2.8 R
11251 35mm F2.8 R
11144 35mm F1.4 R
11216 50mm F2 R
11777 50mm F1.4 R
11253 60mm F2.8 R
11881 80mm F1.4 R
11154 90mm F2.8 R
11254 90mm F3 R

The above are compatible with LECA R and unsuitable for all models of LEICAFLEX (no aperture to-meter coupling). To prevent mismatches these lenses have a modified bayonet mount which only LEICA R cameras accept.
There have always been slight differences between the bayonet mounts of LEICAFLEX SL, LEICAFLEX SL2 and LEICA R to prevent inappropriate combinations.
To offer a service to those who acquire older cameras second hand and wish to extend their outfit with new lenses, arrangements have been made whereby the above lenses (excepting for technical reasons the f/1.4 Summilux-R) can be modified by LWA Service Departments. The modifications comprise a change of bayonet mount and the fitting of Cams 1 and 2. The lenses are then comparable with all LEICA SLR cameras. Enquires should be made of our Service Department regarding costs and delivery times.
Changes have also been made to the front bayonet mounts of BELLOWS-R and EXTENSION TUBE SET 14159. Both items now have the type of mount fitted to LEICA-R camera. Each therefore accepts continued and current lenses/accessories including those lenses above which are suitable for close-up work. The exception is that the extension tubes will not (and did not) accept the 80mm F1.4 SUMMILUX-R However, this is no hardship as this lens is not intended for such an application.
Codes remain unchanged. Existing bellows and tubes can be brought up to date by LEITZ Service Department.
Changes to the MACRO-ADAPTER-R and EXTENDER-R 2X are not required. Both were introduced with the LEICA R camera type of bayonet mount at the front and the matching rear bayonet (as now fitted to the 10 lenses listed previously).
The close up accessories ADAPTERS 14198 and 14262 (for the earlier version of the 60mm F2.8 MACRO-ELMARIT-R and current 100mm F4 MACRO-ELMAR-R respectively) will not be changed. Both items are regarded nowadays as items for the LEICAFLEX models. Although either can be fitted to LEICA-R cameras the MACRO ADAPTER-R is better suited because it accepts any LEICA-R lens.

Manual diaphragm R Lenses, R accessories

Key
1. Static Cam 1 for LEICAFLEX
2. Static Cam 2 for S Land SL2
3. Static Cam 3 for R3 and R4
4. Red Index
Versions
With Static Cam 1 only or 2 only
With Static Cams 1 and 2
With Static Cams 2 and 3
Notes
Static Cams zero the meter to allow accurate stop-down metering. With 'FLEX, Static Cam 1 brings meter to the F 11 equivalent.
Static Cams 1 and 2 may appear to be a continuous surface on some items.

Manual diaphragm Lenses

Although an auto diaphragm offers convenience to the photographer, such a device cannot be fitted to some lenses and accessories because of technical considerations.
As an example, consider the 11960 400mm f/6.8 TELYT-R. This lens is designed to have interchangeable back tubes so that the same lens head can be used either on a LEICA SLR camera or the LEICA M plus VISOFLEX.
Furthermore, with either the R or the M back tube, the 400mm lens head can be interchanged with that of the 560 mm f/6.8 lens.

Flexibility
To maintain this flexibility and incorporate a coupling between camera body and auto diaphragm would create considerable mechanical complexity. This would be both extremely expensive and bulky or of doubtful reliability and is best avoided. Consequently a manual diaphragm is fitted.
Such a device requires the photographer to adjust the lens aperture control ring between maximum opening (viewing and focusing) and any smaller aperture required for the actual photograph. Also, TTL metering must be done with the lens stopped down, that is; set at the working aperture.

Static zeroing cams
The external meter of the LEICAFLEX cannot be used for stop down metering. Therefore lenses with manual diaphragm - and accessories that maintain it - must have the appropriate static cam, otherwise the meter cannot be used. The cam has no movement but as the lens is fitted, it bears on the meter-linked mechanism. This action causes the needle in the viewfinder to adopt the f/11 position to which aperture the lens must be manually set.
Even though all other LEICA SLR Cameras have TTL metering, enabling stop down measurements to be made, a static cam is still required. Previous reference has been made to the intentional over-travel of the cameras' spring-loaded, meter-linked mechanisms. A cam on the lens is required to engage the mechanism (during the latter part of the bayoneting-on action) to 'zero' the meter, i.e. to bring it back to the datum point from which exposure measurements are made. Without the cam, under-exposure results.

Again, different cams are needed to match differences in the camera mechanisms and these are illustrated. The manufacturing policy is as for auto diaphragm lenses and the general situation is as follows:
1965 to 1968: Static Cam 1 only
1968 to 1974: Static Cam 1 and 2
1974 to 1976: Static Cam 2 only
1976 onwards: Static Cam 2 and 3
Conversions from Static Cams 1 and 2 or 2 only to 2 and 3 are usually possible. As these may or may not have been done on second-hand equipment, visual examination (see illustration) is the only reliable means of establishing compatibility with a given camera body.

This page created by Jem Kime (15 Jan 2001) using Leitz material from 'Leitz Photographic Bulletin', Nos. 26 & 27. (Oct. '84 & Jan '85) written by Jeff Griffin of Leica UK. Updated with compilation photostat, c. 1988.