Automatic Exposure

The first SLR cameras with TTL metering were semi-automatic – the needle of a galvanometer indicated how far the shutter speed/aperture combinaison was from the ideal exposure, but it was up to the user to turn the aperture ring or the shutter speed knob to adjust the exposure parameters following the directions provided by the needle.

The next step was obviously to design a camera which would set automatically the aperture value or the shutter speed corresponding to the ideal exposure.

Automatic exposure was first adopted by point and shoot cameras, and by a few amateur oriented reflex cameras.

Before Integrated Circuits capable of controlling a focal plane shutter became available, the simplest way to provide auto-exposure was to keep the conventional mechanical shutter, and to use a servo-motor to link mechanically the match needle of the exposure galvanometer to a series of rods controlling the aperture of the diaphragm. It was easier to implement such a system on a fixed lens camera, which explains why at first auto-exposure SLRs (such as the Mamyia Auto-Lux 35 or the Canonex) did not have removable lenses.

The engineers developing conventional SLRs had a few technical hurdles to pass before they could produce auto exposure SLRs capable of attracting a wider audience of seasoned photographers. The difficulties were cleared at the beginning of the 70s, and two types of automatic SLRs started hitting the market between 1971 and 1973.

Aperture Priority Automatic Exposure

The development of integrated circuits made Aperture Priority (pre-selecting the aperture and letting the camera set its shutter speed automatically) relatively easy to implement. The mechanically controled shutter had to be replaced by an electronically controlled model, but no change was needed on the lens mount -at least for Minolta and Nikon. Aperture priority cameras did not need a delicate linkage between the body and the lens, and could be used even with specialized diaphragm-less lenses (mirror lenses, for instance). Pentax had to adopt full aperture metering – they added a few cams to the 42mm screw mount for the introduction of the first electronic SLR, the Spotmatic ES. Nikon and Minolta launched their own aperture priority SLRs soon after (1971-1972). Olympus followed with the OM-2 in 1975.

Shutter Speed Priority Automatic Exposure

On the contrary, Shutter Speed Priority (pre-selecting the shutter speed and letting the camera set the lens aperture automatically) did not require any change on the shutter mechanism used on previous semi-automatic cameras, but precision mechanics was needed to ensure that the diaphragm would close at the exact value determined by the exposure system of the body.
The automatic diaphragms used in pre-selection lenses – see: 50 years of evolution of SLR lens mounts (Part I of VI) had so far been working in an all or nothing mode: full aperture, or closed down as far as the lens could go, namely to the aperture pre-selected by the user. With Shutter priority, the brute force approach did not work anymore: a lever on the mount (camera side) had to transmit to the lens the exact aperture value determined by the automatic exposure system, and all the lenses of the system had to react identically to the movement of the exposure lever of the camera.

The lenses and the mounts had to be modified again, either by the addition of aperture transmission pins (that’s what Canon did in 1971 with the new FD mount), or by the transformation of the all or nothing stop down command of the diaphragm into something much more linear, where the movement of the stop down lever was proportional to the value of the aperture to be used. Nikon chose the latter route with the AI-S mount introduced progressively after 1979, in preparation of the launch of future cameras offering Shutter Speed Priority and Programmed Exposure modes.

The first Shutter Priority SLR of one of the “big four” was were introduced by Canon (the EF model in 1973) following the tracks of Konica, which had been producing its Autoreflex T series since 1968.

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More about it
A history of the Nikon F mount by Denton Images

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