Autofocus

Point and shoot cameras started adopting autofocus mechanisms in 1977, following Konica’s introduction of the C35 AF.
Pentax and Nikon were the first to show autofocus SLRs with passive autofocus systems (in 1981 and 1983 respectively), but were not successful. For the occasion, both companies had designed a system where the camera body (or prism) housed the focus sensors and the lenses contained the motor used for focusing. Both cameras were using a “contrast detection” autofocus system. Pentax and Nikon had to develop specific variants of their mount (K-F and F3-AF respectively) for the occasion. In the typical Nikon manner, the two lenses designed for the F3-AF will still be compatible with the F4 body launched a few years later.

Minolta and the hybrid design

In 1985, Minolta took a different approach. Using a much more efficient “phase detection” autofocus module, the engineers of Minolta developed a brand new camera system from the ground up. Abandoning the MD mount, Minolta designed a large diameter bayonet mount, and launched the new series of “A” lenses. No compatibility with the old Minolta system was offered, but it did not prevent the Minolta 7000 from being a huge success. It redefined the SLR category, and with the exception of the Canon EOS cameras, all autofocus SLRs would be following the same design principle for a long time.

The “A” mount – which is still used today on Sony SLRs, is a mechanical-electrical hybrid. The lenses have no aperture ring and the transmission of the aperture parameters between the lens and the body is electric, but the diaphragm is closed to its designated aperture by a mechanical lever. Minolta placed the autofocus motor in the body, and had to implement a telescopic shaft mechanism to control the focusing group of the lenses.

The competition was quick to react. Nikon (at the end of 1985) and Pentax (1987) adopted the same hybrid design, but decided to make the new models compatible with their legacy: their AF bodies could still accept non-AF lenses, and their new AF lenses could also be used on conventional SLR bodies.

Canon’s revolutionary EOS system

Canon needed more time than Nikon to react to Minolta’s innovations, but took a much more radical approach. The EOS system, launched in 1987, adopted an all-electrical approach, using an electrical command for the diaphragm and placing the focusing motor in the lenses (as opposed to the camera’s body). The EOS cameras and their lenses used the brand new EF mount, which did not offer any compatibility with the older FL or FD mounts. The absence of compatibility was a shocker for the faithful Canon users, but the new EOS system was obviously designed for the future: Canon did not need to make any change to the mount since its inception, and the all-electric exchange of information between the body and the lens was adopted progressively by all of its competitors.

At the beginning, most of the EF lenses used a conventional electrical motor to control focusing, but a few pro-telephoto lenses benefited from an ultrasonic motor (“USM” in Canon’s marketing brochures). Ultrasonic motors have a very low inertia and operate silently: USM lenses focus very rapidly, and allow the photographer to adjust the focus manually, after the autofocus process has taken place, and without the risk of breaking a cog wheel or the autofocus transmission shaft. USM lenses gave Canon a huge competitive advantage, in particular for sports and action photography.

Nikon recognized rapidly that mounting the autofocus motor in the lens was the right thing to do, and started converting its professional telephoto lenses to an almost all-electrical design (AF-I mount in 1992), which evolved into the current AF-S mount when ultrasonic (“Silentwave”) autofocus motors were adopted (1996). Pentax and Sony were slower to place the autofocus motor in the lens, and most of their current lenses still follow the original hybrid design of Minolta.

Some of the other camera makers decided not to convert their product line to autofocus (Leica, Konica), while some tried but could not get market acceptance (Olympus, Contax). In all cases, the failure to adopt autofocus relegated the camera makers to a niche market, and all ended up stopping production of SLRs altogether.

Almost 25 years after the introduction of the Minolta 7000, it’s interesting to compare Canon’s strategy with Nikon’s. Canon’s users had to absorb a huge change in 1987 (they had to throw away their lenses and buy new ones if they wanted to use the new EOS cameras), but the EF mount and the lenses have remained the same for the last twenty years. All-electrical USM lenses gave Canon a huge competitive advantage, and Nikon, with its more conservative and evolutionary approach, needed almost 20 years to catch up on the professional market.

The state of the art in 1987

Pentax: KAF version of the K bayonet mount. Mechanical control of the diaphragm, autofocus motor in the camera body.

Canon: New EOS mount, 100% electric. Totally incompatible with the FL and FD mounts

Minolta: New A mount totally incompatible with the old SR, MC and MD mounts. Mechanical control of the diaphragm, electric transmission of the required aperture value from the body to the lens, autofocus motor in the camera body.

Nikon: AF version of the F mount. Mechanical control of the diaphragm, electric transmission of the required aperture value from the body to the lens, autofocus motor in the camera. Upward and backward compatibility preserved on professional cameras and lenses (and with limitations on the consumer grade cameras and lenses).

Olympus: new line of lenses, with no focusing ring (autofocus had to be overriden using a rocker switch on the camera body). The reception of the market was so disappointing for Olympus that they gave up on autofocus SLRs, and reverted to conventional OM cameras until 2003.

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