The automatic bodies of the early seventies still required some input from their users: they could only determine the shutter speed (or the aperture in the case of Canon cameras) after the photographer had set an aperture (or a shutter speed) compatible with the film speed, the intensity of the light and the characteristics of the scene (portrait, action shots, macro, and so on).
If the aperture set by the user was too low or too high, a matching shutter speed could not be selected by the camera and the picture was hopelessly under or over exposed.
Similarly, if the photographer let the camera select a very slow shutter speed with a long tele-lens, the picture would be blurry and unusable. Trained photographers knew that. But a better automatic exposure solution had to be found for the photographers who did not want to be bothered with technical details.
Inspired by the program modes already available in point and shoot cameras, Canon launched the A-1, a new SLR with programmed exposure modes in 1978. Practically, it meant that the auto exposure system of the body had to simultaneously command the shutter speed and the aperture of the diaphragm.
Canon did not have to change anything on the FD mount, which had been created for full aperture shutter priority exposure.
Nikon introduced the “AI-S” generation in 1979 when the mount was modified to support a linear command of the diaphragm. The first Nikon cameras to take advantage of the AI-S lenses and to offer a program mode and shutter priority were launched in 1982 and 1984 respectively. Because the camera body was informed of the focal length of the objective, it could choose automatically between two aperture-speed combinations when configured in program mode, one for wide angle and normal lenses, and one for lenses of 135mm and longer .
Still trying to catch up with Nicanolta, Pentax adopted a brand new bayonet mount, the K mount, in 1975. The first K mount, however, did not support shutter priority or program modes. Electric contacts would have to be added with the KA declination of the K mount in 1983 to make it possible. Its close derivatives are still used today on Pentax DSLRs.
The state of the art between 1971 and 1985
Pentax: Aperture priority automatic cameras launched in 1971 with modified 42mm screw mount lenses supporting full aperture metering.
Change from the 42mm screw mount to a new Pentax K bayonet in 1975 (automatic pre-selection, full aperture metering, transmission of the pre-selected aperture value from the lens to the body);
Shutter priority and program mode introduced in 1983 with the KA version of the K mount.
Canon: The FD breech mount introduced in 1971 was ready for the Shutter priority cameras launched in 1973 (Canon EF) and for the program mode (Canon A1, 1978).
Minolta: MD declination of the SR Mount (one pin added for the support of the Shutter priority mode) to support the Shutter priority mode in 1977.
Nikon: Aperture priority cameras available since 1971 (Nikon EL) with the manual indexing F mount. Launch of the AI version of the F mount in 1977 to improve the ease of use. Progressive adoption of the AI-S declination of the F mount in 1979 to prepare for the arrival of cameras offering a program mode (Nikon FG, 1982) and a shutter priority automatic exposure (Nikon FA, 1984).
Olympus: the OM mount was introduced in 1971, and was ready to support programmed exposure from the beginning.
More about the lens mounts