Lens Mounts and Fixings
For use on field and hand cameras with variable bellows extension. The lens is in front of the flange.
Typically for use on reflex cameras. The lens is behind the flange.
SlidingThe lens tube slides within an outer barrel. Often used on small early lenses.
SlotThe lens tube moves within the barrel and is guided by a pin moving within a helical slot. A focusing scale is usually present.
Rack & PinionBritish camera lenses had the pinion mounted tangentially to the lens barrel with only a single focusing wheel. Lenses for projectors and enlargers would often have a focusing wheel at each end of the pinion. In America and on some European lenses a radial fitting was often used.
HelicalThe lens tube moves on a multi-start thread within the lens mount. To prevent rotation of the lens during focusing a collar is normally fitted between the lens tube and the outside mount. The collar fits to the mount by a fine thread and rotates to focus, the lens fits to the inside of the collar by the multi-start thread, studs on the lens tube move in slots in the outside mount which prevents the lens from rotating but allows linear movement.
Lens ElementWith some lens designs altering the separation of the lens elements will alter the focus (focal length) of the lens. This may be used to focus the image rather than altering the lens to film distance. This was proposed by William Taylor using a Cooke Triplet with a movable front element, the lens was advertised by Taylor, Taylor & Hobson in 1903. A very similar patent was taken out by A.L. Adams and William Watson 1
Close-up lensesA focusing method popular on box and detective cameras was to have a fixed lens and use close-up lenses to focus at near distances. The Frena camera had sets of three or four separate lenses that attached to the front of the lens. Other cameras had close-up lenses on a rotating wheel or slide fixed to the camera.
Fixing to the Camera
In Britain, from the 1870s onwards, the lens flange was usually attached to the camera with three screws except for larger models which used more. Earlier cameras, even of small size, typically had the lens flange attached by four screws. Most manufacturers used countersunk screw heads to attach the flange, an exception was Lancaster who used dome-headed screws.
The usual method of attaching the lens to the camera was a screw thread, standard thread sizes were proposed by the Royal Photographic Society (RPS) which were adopted by some manufacturers including Taylor, Taylor & Hobson (T.T.H.) and, from around 1890, Ross. The RPS sizes, first proposed in 1881 and later modified, used a Whitworth Angular thread, for small diameters 24 threads per inch was used for sizes above 3" 12 tpi.2
An important development was made by T.T.H. in 1892 by chamfering the start of the thread which made picking up the thread much easier.3
This was occasionally used, it made lens changing quicker but did not prove popular. The Newman & Sinclair Patent Reflex of 1911 was an early example.
Many field cameras had small interchangeable lens panels held in place by turn screws, the Improved Long Focus model by Billcliff (c. 1885) had a bayonet fitting lens panel. Removable lens panels are usually present on hand and stand cameras and later technical cameras.
A simple bayonet consisting of two studs on the lens matching slots on the flange was used on the Kinegraphe of 1887, Thornton-Pickard advertised similar lenses at the same time.4
Newman & Guardia used a bayonet fitting on the Special B (1895) for the front lens of the Double Protar. On some of their cameras the whole lens mount was also attached by a bayonet mount.
Bayonet fittings were sometimes used on folding plate cameras from the 1920s and 1930s.
A criticism of bayonet mounts was that friction between the lens/camera surfaces would alter the alignment. On a breech lock the lens pushes into the camera mount and is locked by turning a ring that pulls the two surfaces together. The ring acts on another part of the mount not the mating surfaces. The ring can be either on the lens or the camera.
When several lenses were used with a camera they might not fit the same flange. Stepping rings could be used to reduce the flange size if the difference in size was small. Universal mounts were also available, though not popular, with these the lens was held by a set of thin plates much like an iris diaphragm.
In the era of the field camera only the physical attachment of the lens to the camera - does the screw thread match the flange - had to be considered. With the introduction of miniature cameras the type of mount and the flange to focal-plane distance have become distinctive of a particular camera. A number of linkages between the lens and camera have also developed.
The first was for rangefinder coupling where, typically, a spring-loaded lever in the camera, operating the rangefinder optics, pressed against a surface on the lens. Common arrangements were:
- To have the surface in the lens mount shaped as a cam that revolved as the lens was focused.
- To have the lever rest against the end of a ring within the lens. The ring was loosely connected to the focusing mechanism but was separately threaded and so could move at a different rate.
- A ring threaded to the inside of the focusing movement had an extension piece that moved within a slot in the lens mount or flange and so was forced to move axially as the focusing movement was turned. An arm attached to the ring engaged the rangefinder lever. This version is most suitable for long-focus lenses.
The focal-length of the lens may also be passed to the camera so that the correct view-finder mask is used.
The next linkage was to operate an automatic or semi-automatic diaphragm, this usually consisted of a pin projecting from the lens which was depressed by a plate in the camera during the exposure. Typically the iris is pulled open by a spring in the lens, when the pin is depressed by the plate the iris closes to its pre-set position. The reverse arrangement is also found where the iris is pulled shut by a spring and opened by the plate in the camera.
Normally the spring holding the diaphragm in its default open or closed position is within the lens, an exception was the Zeiss Contarex where the aperture is set on the camera not the lens. The lens itself has a manual diaphragm operated by a ring inside the bayonet mount, the ring is engaged by a second ring in the camera which rotates to open the diaphragm when the shutter is set and to close it when the shutter is released. When the lens is mounted on the camera a cut-out section of the lens mount moves a lever connected to the exposure meter to set the maximum aperture of the lens within the meter.
With the advent of coupled exposure meters more linkages were necessary. In 'full-aperture' mode altering the diaphragm setting is simulated in the meter, the diaphragm itself remains open. The meter also needed to find out the maximum aperture of the lens, either directly from the lens or in a semi-manual way as on early Nikons. Cameras with an automatic 'shutter priority' setting had to set the aperture on the lens.
Scales and Fittings
Depth-of-Field values were often marked on focusing lenses they usually took the form of a printed scale next to the focusing scale. Another arrangement, common on lenses fitted to Compur shutters, was to indicate the depth-of-field by two arms that moved as the aperture was altered. Kern displayed the values in small holes that changed colour as the aperture was changed.
On lenses with fully automatic diaphragms there is normally a switch to set the diaphragm to manual. A depth-of-field preview button may also be present.
Several cameras from the 1960s had a 'shutter' release on the lens which fitted in front of the normal release on the camera body providing a fully automatic diaphragm.
A cable release socket is sometimes fitted to be used with a double cable release when using close-up accessories.
References & Notes
 Taylor, BP 6029/1900. Adams, Watson, BP 8099/1900.
 BJA 1900, 1123. BJA 1902, 1125a.
 BP 3019/1892.
 YBP 1888, p. xcvi.
 In a simple rangefinder such as the swivelling prism type, the angular deflection of the prism is nearly uniform with the linear movement of the lens. With a 35 mm camera fitted with a 5 cm lens, the prism is deflected through an angle of around 3° between infinity and 1 metre, the movement of the lens was around 3 mm.
The rangefinder is calibrated to a standard lens of precise focal-length, this might be different for lenses that have the same mount and appear to couple correctly, the coupling will diverge as the lens is focused on nearer distances (e.g. the incompatibility between Contax and Nikon lenses). In the Summicron lens from Leitz the optics were grouped into one of three focal-lengths (51.6, 51.9 and 52.2 mm), three focusing mounts were available to match the lens head (the problem arose because the user could purchase a lens head separately to the lens mount).
Lipinski, p. 81 shows a rangefinder coupling.