The motors that drive a camera's autofocus system have changed dramatically since the first one in 1985
In the first really successful autofocus system, which was fitted to the Minolta 7000 in1985, the motor that drove the focus scroll of the lens was in the camera, with a mechanical ‘screwdriver’ link from camera to lens. As other manufacturers introduced their own autofocus systems, they generally followed that same architecture. Nikon, Yashica and Pentax produced similar ‘screwdriver’ systems, with the only major manufacturer breaking ranks being Canon, whose designers decided that the right place for the autofocus motor was in the lens.
Over the next 30 years or so, it became clear that Canon’s designers had got it right, and the other manufacturers converted their systems to follow the Canon architecture very closely.
Canon’s release of the EOS (Electro Optical System) in 1987 included two different types of motor. The inexpensive lenses had standard permanent magnet DC motors, connected to a gear train, that were designed to wrap around the optical cell of the lens. The expensive lenses were fitted with a novel piezoelectric ring motor. This type of motor had three advantages. First, it provided high torque at low speeds, so you could drive the focus scroll of the lens without a gearbox. That fact was responsible for the second advantage, because without the whirring gears the motor was virtually silent. Third, the motor was shaped like a ring, and it was large enough for the optical cell of the lens to fit in the middle, resulting in a relatively compact package.
Those two motor types formed the basis of all in-lens autofocus systems for the first 30 years of autofocus cameras, and then suddenly two new types of focus motors entered the market. Video was the driver for both. Video requires two things from a motor: quietness as well as smooth and controllable ‘focus pulling’. The old geared DC motor failed to meet the first requirement, while the piezoelectric ring motor had difficulties with controllable motor speed. Panasonic was the first to introduce lenses that included stepper motors. A stepper motor is a synchronous motor, as you’d find in an old-fashioned electric clock. When the AC wave forms feeding it are computer generated, it can be made to move in a precise manner. They can also be made to produce high torque, minimising the need for whirring gear trains. Stepper motors are not new – they have been the mainstay of inkjet printers.
The second new kind of motor is the Linear Electromagnetic Motor (LEM), which was introduced by Sony and Fujifilm and is found in the Zeiss Batis lenses. This is the loudspeaker voice coil. In the LEM motor, the conventional focus scroll is abandoned and the focus cell is directly driven backwards and forwards in the same way that a loudspeaker cone is. The advantage is fast, silent and controllable movement. The disadvantage is that with no focus scroll, there is no way that these lenses can provide direct manual focus. Instead, they provide ‘focus by wire’, where the movements of the manual focus ring simply result in commands to the lens’ CPU to move the focus motor.
Bob Newman is currently Professor of Computer Science at the University of Wolverhampton. He has been working with the design and development of high-technology equipment for 35 years and two of his products have won innovation awards. Bob is also a camera nut and a keen amateur photographer