Lens Mounts and Fixings

Lens Mounts

Normal

For use on field and hand cameras with variable bellows extension. The lens is in front of the flange.

Sunk

Typically for use on reflex cameras. The lens is behind the flange.

Focusing

Sliding

The lens tube slides within an outer barrel. Often used on small early lenses.

Slot

The lens tube moves within the barrel and is guided by a pin moving within a helical slot. A focusing scale is usually present.

Rack & Pinion

British 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.

Helical

The 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 Element

With 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 lenses

A 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.

Screw Thread

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

Interrupted Thread

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.

Lens Panel

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.

Bayonet

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.

The image, right, shows a bayonet mount used to attach the front group of a combinable lens. The image, far right, shows a typical bayonet fitting from the 1950s on an Agiflex.

The image, right, shows the bayonet mount of an early Exakta. The image, far right, shows the two bayonets fitted to the Contax. The inner bayonet, within the focusing bezel, takes standard lenses, others fit to the outer bayonet.

Bayonet fittings were sometimes used on folding plate cameras from the 1920s and 1930s.

The image on the right shows a Bergheil camera.

Breech

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.

Universal Mounts

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.

Interface

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 image on the right shows this type of coupling on a long-focus lens.

The rangefinder in the camera was calibrated for a lens of standard focal-length. When a cam was used, lenses of other focal-lengths had a differently shaped cam. With the other options listed above, the ring would translate the focusing movement into the equivalent movement of a standard lens by working in a thread of different pitch.⁵

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.

The image, right, shows a diaphragm activation pin and below it a depth-of-field pre-view lever.

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.

The image on the right shows the iris setting ring on a Contarex.

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.

The image, right, shows the cam on a Leicaflex that is positioned as the diaphragm ring is turned, a feeler in the camera rests on the cam to detect the aperture. A bar in the camera presses down on the Aperture Activation Lever which keeps the diaphragm open, when the shutter is released the bar is raised allowing the diaphragm to close to its pre-set position.

The image, far right, shows a Canon FD mount. Full Aperture Signal Pin - indicates the lens's maximum aperture. Aperture Signal Lever - indicates to the camera the value set on the aperture scale. Automatic Aperture Lever - sets the aperture to the pre-set value. AE Switch Pin - Indicates whether the lens is set to Automatic. Positioning Pin - Aligns the lens to the camera body.

Scales and Fittings

Focusing

To focus a camera either the focal length of the lens is altered or the distance between the lens and the film plane is altered. Altering the lens focal length by employing weak positive lenses or by moving the front element is dealt with above.

On cameras with helical lens mounts the focusing scale is usually on the lens itself. On cameras where the front standard is moved (e.g. Hand & Stand cameras) there is usually a scale on the folding bed or camera body.

Some cameras could be fitted with different film/plate holders these may have different film planes e.g. using a Film Pack Adapter or fitting dark slides to a roll film camera. The focusing scale might be printed with two sets of values or be moveable; on N&G Sibyl cameras the carriage holding the front standard was moveable and clipped into one of two positions (marked for dark slides or film packs). On the Goerz Tenax a negative lens attachment was fitted when a film pack was used. A similar problem arose when Autochrome or similar colour plates were used where the plate was reversed in the dark slide, cameras could be marked with two scales (such as the Verographe), or a negative lens attachment could be used such as the Zeiss Ducar; where a focusing screen was fitted the easiest approach was to reverse the screen.

When focusing on a near subject the lens has to be extended from the infinity position a distance d = FxF / u - F, where F = focal length, u = distance of subject from the lens.

The image far right shows the focusing scale marked for different lenses on a Mamiyaflex. The image right shows the dual scales, colour and monochrome, on a Verographe.

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.

The image, far right, shows the depth-of-field scale on a lens from an Edixa, the red 'A' in the centre of the scale indicates that the diaphragm is set to automatic it changes to 'M' when the diaphragm is switched to manual, the small red dot at f5.6 is the infra-red focusing index. The aperture is displayed in a small window at the bottom of the lens.

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