When we look at any photograph, one of the first things that we notice is whether it is in focus or not. While there are some exceptional photographs that stick in the mind despite being out of focus, the aim of almost all photographers universally is to record the subject sharply on film or on the digital sensor.

In the early days of autofocus photography (Canon’s first AF SLR body was the T80 in 1985), the AF drive motor was placed in the camera body and drove the lens mechanically. With the introduction of the EF lens mount in 1987, the fully electronic connectors allowed the autofocus motor to be moved from the camera body to the lens itself, giving the possibility that each AF motor could be tailored to suit the lens it was fitted into, thereby providing faster autofocus.

 

The three types of USM focus motor showing where they fit in the lenses.

Since the AF motor is now closer to the lens, it can be made more efficient and quieter. The other advantage of the in-lens AF motor is that as new AF motors are developed, they can be incorporated into lenses and will work with older Canon SLR cameras that feature the EF lens mount.

Although the decision to move the AF motor into the lens had been made, there was still a need to create a motor that could work well and satisfy the aims of fast, smooth and quiet autofocus. The result was the EF300mm f/2.8L USM lens launched at the start of the EOS system in 1987. It featured a ring-type Ultra Sonic Motor (USM) motor that was both fast and near silent. In 1990, new manufacturing techniques made it possible to reduce the cost of manufacture and ring-type USM motors found their way into Canon lenses at a consumer price level.

Two years later, in 1992, automated production lines led to the development of the Micro USM motor for use in consumer lenses. Ten years after that, in 2002, came the Micro USM II motor, which is only half the size of the original Micro USM motor. The long-term aim for Canon is to introduce USM motors in every lens in the EF lens range. In fact, almost all current lenses fulfil this desire, with just a few consumer end lenses featuring non-USM lens motors.

As we’ve seen, there are three types of USM motor - the ring-type, Micro and Micro II types. Like the AF motors from other manufacturers, the USM motors aim to convert an electromagnetic force into a rotational force to drive the lens-focusing group. What is different about USM motors is that they use ultrasonic vibration energy which is converted in the rotational force.

Ring-type USM

 

Ring-type USM motors showing the simple construction and arrangement of the rotor and stator. The stator is the toothed ring at the rear.

The ring-type USM motor is the most widely used AF motor in the Canon EF lens range. Of the 49 Canon lenses that use USM motors, 42 of them feature a ring-type USM.

To be effective, the ring-type USM motor needs to meet certain requirements. They must be powerful enough to drive the focusing lens group quickly and easily at low speed so as to avoid the need for a gear system to reduce the speed. They must exhibit high levels of holding power so that once the motor is switched off, the focusing lens group is held in place without any further input needed. They should be simple to manufacture and should start and stop quickly to ensure the best focus response. They should also be as quiet as possible in use.

In addition to these features, ring-type motors are also highly efficient and feature a low power consumption to extend the camera battery life. Being ring-shaped, they are ideally suited to the shape of the lens barrel, their focusing speed is very controlled and they are stable across a wide range of temperatures, from -30degC to +60degC.

The ring-type USM is actually very simple in operation. It is composed of a rotor and a stator – an elastic body with a piezo-electric ceramic voltage element attached to it. By applying an A/C current with a resonant frequency around 30,000Hz to the stator, vibrations are created causing the rotor to rotate continuously. 30,000Hz is in the ultrasonic range, and this is where the USM motors derive their name.

The piezo-electric ceramic element is split into two phases – A-phase and B-phase. By alternating this resonance between these two phases, rotational force is created which can be used to rotate the focusing group and so focus the lens.

Micro USM

 

Cross section of a ring-type USM motor fitted into the EF28-135mm f/3.5-5.6 IS USM lens.

While the ring-type USM is designed to fit in the circular barrel of a lens, the Micro USM motor was created to fit in a wide range of lenses without being restricted to the size of the lens barrel. Unlike the ring USM where the stator and rotor are separate parts, in the Micro USM motor, the rotor, stator and drive gear at combined into one unit roughly half the weight of a ring-type USM motor.

The Micro motors are also cheaper to produce making them more suitable to the consumer range of lenses where cost is an issue. In principal, the Micro USM works in a similar way to a ring-type USM, with ultrasonic vibrations created by piezo-electric elements. There are four piezo-electric layers, which are each constructed from two alternating phase piezo-electric elements. These elements are offset from each other in alternating phases by 90degrees.

By applying an A/C current to only the A-phase, the stator will vibrate left and right. If current is applied to the B-phase, the stator will rotor forwards and backwards. When current is applied to both the A-phase and the B-phase the resulting motion is rotational as the tip of the stator moves, for example, left, back, right, forward, left, back, right, forward. This rotational force is applied to the main drive gear, which in turn is used to drive the gears of the focusing mechanism.

 

On the left of this picture is the Micro USM motor and on the right the Micro USM II motor – this illustrates the relative size differences.

Micro USM II

The Micro USM II motor is essentially a reduced size version of the Micro USM motor. It functions in a very similar way but the length of the unit has been greatly reduced to allow it to be used in ultra-compact zoom lenses. The reduction in size has been achieved by re-arranging the rotor and stator so that instead of being aligned in a row, part of the stator is placed inside of the rotor. This required the creation of a new format of vibration so that the resonant frequency of the piezo-electric elements was not too high resulting in insufficient vibrational amplitude.

The result of this re-arrangement is that the Micro USM II is about half the size and half the weight of a Micro USM motor, and yet retains almost the same performance characteristics. The first lens to feature the Micro USM II was the EF28-105mm f/4-5.6 USM that was launched in 2000. Since it is so well suited to use in the compact zoom lens range it is likely there will be more than just the current two lenses featuring it in the future.

 

The new Canon EF200mm f/2L IS USM lens incorporates a ring-type USM motor.

Conclusion

Since the launch of the USM motors with the EOS system in 1987, the technology has advanced enormously. While we still have the ring-type USM motor, and it is still the best AF motor available, we now have USM motors in lenses it would previously have been impossible to include them in due to size and cost issues. By having three USM AF motors to choose from, it allows the designers to select exactly the right type to suit the needs of the lenses. It will surely not be long before all of the autofocus lenses in the Canon range feature a USM focus motor helping you to get sharp, in-focus images time after time.