50 years of lens mount evolution (Part I of VI)

Nikon was very proud a few months ago when the 50th anniversary of the F mount was celebrated. Half a century! Pentax had to abandon its orginal mount and transition to a new bayonet in the early seventies, Minolta and Canon in the mid eighties.

Nikon F - Photo courtesy of cameraquest (www.cameraquest.com)
Nikon F - Photo courtesy of cameraquest (www.cameraquest.com)

But there is more to lens and body compatibility than the design of the bayonet.

Even if the current Nikon bodies and lenses still use the same bayonet design as the Nikon F of 1959, it’s practically impossible to pair an unmodified lens from 1959 to a recent body, and vice versa: the lens and the body of a modern SLR have to exchange information and commands, and non-upgraded lenses from 1959 simply don’t share enough information to be usable.

The transmission of information from the lens to the body – focal length, maximum and minimum aperture, pre-selected aperture, focusing distance, and of commands from the body to the lens – setting the focusing distance, setting the aperture value, closing the diaphragm, can be performed from many different ways – some of them passive (a hole in the metal), some of them mechanical (rods, cogs and springs), the most recent working exclusively through electrical contacts.

Diaphragm pre-selection

Cameras of the mid fifties were far less complex than the ones we now use. No internal meter, no auto exposure, no autofocus.

But users of SLR cameras were facing an important issue: because the viewfinders of their cameras were dim and the focusing screen grainy, the only practical way to set the focus was to open the aperture to its maximum. Let’s say F:1.4. But if on a sunny day they needed to shoot at 1/125 sec at F:11, they had to set the aperture ring to F11 AFTER they were finished with the focus and – of course – BEFORE they took the picture. Not very fast, not very convenient.

At the end of the fifties, most Japanese camera manufacturers adopted automatic diaphragms with aperture pre-selection: the lens remained at full aperture – let’s say F:1.4 -independantly from the aperture value selected by the user on the aperture ring, making focusing easy. Only when the user pressed the shutter release to take the picture would a lever or a rod mechanically close the diaphragm to the value pre-selected by the user.

Nikon F Mount - the stop down lever is at the top to of the picture
The stop down (diaphragm preselection) lever of the camera is visible at the top of the picture. It controls a corresponding lever on the mount of the lens

The Nikon F Mount - the stop down lever is closing the diaphragm when a picture is taken
When the shutter release is pressed, the stop down lever of the body activates the corresponding lever on the lens to close the diaphragm at the pre-selected aperture (Nikon FM)

With the introduction of the automatic diaphragm and the instant return mirror, SLRs had become usable for action photography, and the Nikon F was ready to replace rangefinder cameras as the tool of choice of photojournalists.

The state of the art in 1960 (leading Japanese manufacturers)

  • Asahi Pentax : 42mm screw mount, automatic preselection on some lenses
  • Canon: R series breech mount with automatic preselection
  • Minolta: SR series bayonet mount, automatic preselection on some lenses
  • Nikon: F mount bayonet, automatic preselection

It is worth noting that even the most recent Nikon lenses and bodies still use the diaphragm preselection mechanism introduced with the Nikon F in 1959.

The Nikon F Mount - the AI-S variant (left) and the AF-S (right). Mechanically identical.
The Nikon F Mount - the AI-S from the seventies (left) and the current AF-S (right) variants - this AF-S lens has 10 electrical contacts which are absent from the AI-S, but both still use the stop down lever (on the left of the mount) for the control of the diaphragm

Sources of information about cameras from the fifties and sixties

Photography in Malaysia (Mir Photography)
SLR Lens Mount Identification Guide