Professor Newman on…
Professor Robert Newman explains the various types of autofocus motors commonly used by lenses
Over the past few years, the variety of focus motor systems, all with impressive and sometimes even poetic branding, has burgeoned. So, if you have difficulties differentiating between a Silky-Smooth Voice coil Motor and a Rapid eXtra-silent stepping Drive and you’d like to know how they work, then this article should be of interest to you.
The first three autofocus SLRs, the Pentax ME-F(1981), the Nikon F3AF (1983), and the Canon T80 (1985), all had focus motors built into the lens. This made the lenses bulky and unwieldy, and none of them caught on. Alone amongst the manufacturers, Canon persisted with the in-lens motor. The design problems here are to do with torque. Driving torque is how much twisting force a motor can apply, while holding torque is how much force can be applied to a stationary motor without it moving.
A conventional electric motor has low torque relative to its size. The usual solution to this is a gearbox, which converts from high speed/low torque to low speed/high torque. It was in part the gearbox that had made previous AF lenses so bulky.
In the lenses for the 1987 Canon EOS 650, this was solved using two technologies. The first was ‘Arc-Form Drive’ (AFD), simply a cleverly packaged gearbox, which could fit within a cylindrical lens. Canon’s more consequential advance was the ultrasonic motor (USM). This was a different form of motor with the required high torque/low speed characteristics. A ring of a piezoelectric material (that changes shape when subjected to an electric charge) is sandwiched between two metal rings. By providing a high frequency modulated charge, the piezo ring can be made to flex into a wavy shape. This can be made to move around the ring, moving the relative position of the outer metal rings, and therefore turning the focus scroll.
The ultrasonic motor was adopted by every manufacturer, using brand names such as ‘Hyper-Sonic Motor’ and ‘Silent Wave Motor’. Sometimes it is packaged as a small discrete motor, similar in form to a normal electric motor, which drives the scroll via a small gear. Typically the focus scroll is attached via a differential gear working against the manual focus ring, allowing manual focus whilst the AF is still operational, made possible by the high holding torque of the ultrasonic motor.
In digital cameras, there is no real possibility of operating the camera without a battery. So manual focus can be achieved using ’focus by wire’, whereby the focus r
