The introduction to Through The Lens (TTL) Light metering and its consequences on the lens mount
Now that the instant return mirror and the preselection mechanism of the diaphragm had made SLRs usable for action photography, the manufacturers managed to address the next challenge: the determination of the exposure.
Obviously the solution was to install the light meter INSIDE the body, behind the lens, and to capture the exposure parameters (shutter speed, pre-selected aperture, film sensitivity) from the inside of the camera, without any intervention from the photographer.
At the beginning of the sixties, smaller and more sensitive light meters, using CdS (Cadmium Sulfite) sensors, became available. Pentax and Topcon compete for the honor of showing and producing the first cameras with Through The Lens (TTL) metering. The Pentax Spotmatic was shown as a prototype in 1960, but Topcon was the first to launch a TTL camera in 1963 (the Spotmatic only reached volume production in 1964).
Those cameras worked in a semi-automatic mode: the user had to turn the aperture ring or the speed knob to position a needle controlled by the light meter on an index in the view finder (the matching needle system).
In the Pentax and Canon (FL mount) implementations, the light meter was activated when the photographer pushed a metering lever, which closed the diaphragm at its pre-selected aperture in what is called Stop Down Metering. It made the implementation simpler – no need for a linkage between the lens and the camera body, but it represented an unnecessary complication for the photographer, and darkened the viewfinder just when he was about to finalize the composition of the picture. There had to be a better solution.
Other manufacturers chose to keep the lens at full-aperture during the light metering, and not to close the diaphragm at its pre-selected working aperture until the shutter release was pressed. But it meant that a mechanical system had to transmit the value of the pre-selected aperture from the lens to the light meter.
When they implemented full aperture metering, most camera manuufacturers (Canon with the FD mount, Minolta with the MC mount, for instance) opted for a transmission mechanism internal to the lens, even if it meant adding aperture indexing pins to the lens and changing the design of the mount once again.
Nikon developed an external system: a prong often nicknamed “the rabbit ears” and located on the aperture ring of the lens would be coupled with a metering pin on the body of the camera. The prong could very easily be retrofitted to the first F mount lenses, preserving compatibility, but it obliged the photographer to follow a cumbersome “manual indexing process” to calibrate the lens with the body after each lens permutation. The indexing process was simplified in 1968 when a new generation of Nikon bodies introduced “semi-automatic indexing”, and became totally transparent with the adoption the Automatic Indexing (AI) declination of the F mount in 1977.
AI lenses kept the traditional F Mount bayonet, but used a notch at the back of the modified aperture ring to transmit the pre-selected value of the aperture from the lens to a spring loaded rotating ring on the body . The older F lenses could be retrofitted and would become AI’d lenses. A few camera models (such as the FM and FE) would be capable of using pre-AI as well as AI lenses, but very rapidly only AI and AI’d lenses were compatible with new camera bodies. [interestingly, the idea of using a tab at the back of the aperture ring of the lens to transmit the preselected aperture value to the camera was implemented in 1972 by Fujica, when they introduced the ST801 and the EBC-Fujinon lenses].
The state of the art in the sixties
Pentax: 42mm screw mount, with automatic preselection and stop down metering until 1971.
Canon: FL breech mount with automatic pre-selection, stop down metering (1964-1971)
Canon: FD: evolution of the FL mount with internal transmission of the aperture value to the camera body. Full aperture metering (1971-1987)
Minolta: MC version of the SR bayonet (the same bayonet as before; light meter coupling added): 1966-1977
Nikon: same F mount bayonet with automatic preselection and full aperture metering; external coupling prong (“rabbit ears”) on the aperture ring of the lens for the transmission of the aperture value to the light meter of the camera.
Manual indexing process: 1965;
Semi-auto indexing introduced in 1968;
Automatic Indexing and transmission of the aperture value to the body on the AI lenses by a tab at the back of the aperture ring of the lens, launched in 1977.
Nikon proposed a conversion procedure for the lenses built before 1965 which could be retrofitted with “Rabbit Ears”, and for pre-AI lenses which could be converted to Automatic Indexing by changing the aperture ring.