The lens mount that today is widely known as the Contax/Kiev1 mount was originally introduced together with Zeiss Ikon’s 1932 interchangeable lens rangefinder – the original Contax2. The Contax itself was designed to compete head–on with the Leica cameras which – since their original introduction in 1927 – had already attracted a significant following.
Instead of simply copying the Leica’s approach, Zeiss Ikon set out to improve on the original, and one thing they considered paramount was to devise a rangefinder coupling mechanism that was neither prone to mis-calibration or manufacturing-tolerance induced imprecisions. In short, they set out to devise a mount that allowed for a more precise rangefinder coupling, and they managed to achieve this in an ingenious manner.
At this stage, if you’re not familiar with the Contax/Kiev mount, I recommend you take a detour and have a look at the JAPB article on the Contax/Kiev mount.
First: What is a coupled rangefinder?
A rangefinder – and every artillerist and gunner knows this – is a contraption used to ‘find’ or determine the ‘range’ or distance of an object. Typically range finding is based on triangulation – taking a heading to an object from two different points and using the difference in angle to determine a distance to the object. Rangefinders in cameras are usually optical, which means that they look at an object in the centre of the frame from two points/windows along the top of the camera, and using a set of adjustable mirrors to project a part of the second image onto the first, so that when the pictures match, one can determine a distance. Rangefinders must always be calibrated, so that the double images of an object at 10 metres match exactly when the rangefinder says 10 m. Rangefinders can be had also as simple add-ons to viewfinder cameras (such as this classic example) and cold-shoe mounted rangefinders are still being manufactured.
What you get when you add such a separate rangefinder to a camera such as my trusty 6×6, anno 1930s folding camera (a viewfinder) is an uncoupled rangefinder. In this process, the photographer would set exposure based on whatever logic (either the sunny 16 rule or using a light meter), and would use their rangefinder to determine a distance to the subject. But because that rangefinder is not connected (or, in photo-lingo: coupled) to the lens, the photographer would need to take the reading of the rangefinder, and manually set the lens to focus at that distance.
The, rather revolutionary, idea of coupled rangefinders is that you would connect the rangefinder not to a dial that shows the correct distance to focus your lens at, but that you would couple the rangefinder and the lens, directly. In such an approach, you could either turn the lens’ focus ring, and your rangefinder would change to show how off your focus would be, or turn the rangefinder adjustment ring/wheel, and your lens would automatically change focus. Presto!
While this sounds like a nifty way to make the photographers’ jobs easier and improve their keeper rate, this did tend to add a failure point, in that not only did you need to calibrate the adjustable mirrors, you also needed to calibrate the rangefinder-lens connection.
The obvious way for an interchangeable lens coupled rangefinder (how Leica did it)
Leica had already kind-of committed themselves to using a screw mount – a 39 mm diameter thread with a 26 turns per inch thread pitch – in how to allow for interchangeable lenses, and in such situations, there is one obvious way for a camera to ‘read’ the distance at which a lens is focused, namely by ‘reading’ a cam (sort of a sleeve inside the LTM mount). That reading would then be used to set the rangefinder’s distance, allowing the photographer to check whether their subject would be in focus.
With a standard (FFE ≈50 mm) lens, the number of millimetres the objective needs to move between focusing on infinity and focusing on MFD (≈1 m) was 3 millimetres and that was taken as a baseline. Non-standard objectives thereby necessitated a secondary helicoid that would make sure that the cam would move the same distance between MFD and infinity even when the objective’s real motion would be far more (tele lenses) or far less (wide-angle lenses). Brilliant!?
Problematically, if the entire range of linear movement of that cam is ≈3 mm between Infinity and MFD, this also means that the cam would move by only a minuscule fraction of a millimeter between a set focus on, say, 3 metres and 5 metres. This minuscule movement range combined with pre-war machining standards and the propensity of even minor knocks getting the calibration out of kilter led to that many an image turned out to be out of focus (even though the rangefinder was bang-on) to the great chagrin of both photographers and Leica mechanics.
The Contax’s solution
The engineers at Zeiss Ikon took a radically different approach. Instead of using a cam that moved (by necessity) a limited amount in the direction of the lens’ optical axis, they looked at the rotational motion used when focusing lenses, and thought: “…what if we could use that…?”
So instead of dividing the distance between MFD and infinity into mere millimetres of linear motion, the Contax’s coupled rangefinder was linked to ≈ 330 ° of angular motion. Moreover, in a real stroke of genius, they decided that that same angular motion could be used for standard lenses not only as a measurement of focusing distance, but to actually facilitate focusing. This is the reason for why Contax rangefinders use the so called ‘internal bayonet’ for mounting standard lenses (where the internal bayonet can actually do the focusing motion), and an external bayonet for wider and narrower lenses (where focusing motion would have to be dependent on the lens having a helicoid), but still in a way where it should be coupled to the rangefinder.
Why is this ‘genius’?
Firstly, whereas the Leica thread mount approach always needed a secondary helicoid with <> 50 mm focal length lenses, because you needed a system that would translate the lens’ real focusing motion between MFD and infinity (more than 3 mm for > 50 mm lenses; less than 3 mm for < 50 mm lenses) into precisely 3 millimetres for the rangefinder, whereas in the Contax’s approach you simply needed to define that the focus throw of all lenses would be the same (≈ 330 °) between MFD and infinity, and the actual amount of movement of the objective could be handled by using a steep or shallow helicoid. In a Contax’s rangefinder, all lenses (no matter the focal length) would always be focused at the same distance when turned the same number of degrees from infinity. As a result, the Contax not only allowed for a more precise rangefinder coupling, but there was one less thing that could go out of whack (and would need recalibration).
Second, because one could utilise the internal bayonet for standard (≈ 50 mm) lenses, that meant that standard lenses needed not have a helicoid in themselves, but that the focusing motion could be taken care of by the internal bayonet mount and its integrated helicoid. This, in turn, meant that Contax standard lenses needed no helicoid. None.
That image above (with two Lego minifigs for scale) is a standard lens for a Contax rangefinder mount. Moreover, it’s not any old standard lens, it’s a f/1.5 Carl Zeiss Jena Sonnar, which – when it was introduced in the 1930s – was one of the fastest photographic lenses ever manufactured. And it’s tiny! If that is not genius, I do not know what is.
One way to ascertain whether a solution is good, is to see if others copy it. Hence, the fact that the Soviet camera industry used this approach well into the 1980s should be seen as a confirmation of the Contax’ solution’s viability. But for those who do not think that the Soviet industry’s decision to copy something is an objective endorsement, there is another, likely more compelling exemplar: The Nikon S mount.
The Nikon S mount is an almost 1:1 copy of the Contax/Kiev mount. The principle is the same, the mechanic logic is the same, and you can even cross-mount Nikon S, Contax and Kiev lenses, but with one hitch: Nikon S and Contax/Kiev cameras have slightly different measurements, leading to that if you cross-mount these lenses, the viewfinder calibration will be off by a few percent. While this might not be noticeable except when shooting at larger apertures (shallower depths of field), it has historically been seen as a no-go-zone.
But when using these lenses adapted, you are not actually using the rangefinder mechanism, meaning that that historical no-go-zone is no longer a problem.
Adapting Contax/Kiev (or Nikon S) lenses…
See the JAPB article on the Contax/Kiev mount for details.
1 The reason why the Contax rangefinder mount is nowadays commonly referred to as the Contax/Kiev mount is that while manufacture of Contax rangefinders ended in 1960, the Soviet copy of the Contax – the Kiev rangefinder – continued until well into the 1980s. This leads to that most serviceable Contax rangefinder bodies and lenses are today Kiev cameras and Soviet lenses made for the Kievs.
2 The name ‘Contax’ is legendary, and has since been recycled in products that are both technically and administratively far removed from the original Contax rangefinder.