Canon lens focusing motor technology

When you look at a photograph, one of the first things you notice is whether it is in focus or not. While there are some exceptional photos that stick in the mind despite being out of focus, getting the subject sharp is the aim and starting point of almost all photography.

In the early days of autofocus photography (Canon's first SLR with AF was the T80 in 1985), the AF drive motor was frequently located in the camera body or attached to the lens and drove the lens mechanically. In 1987, with the introduction of the EF lens mount and its fully electronic connectors, Canon was able to miniaturise the autofocus motor to fit inside the lens itself. This raised the possibility that each AF motor could be optimised for the lens it was fitted into, thereby providing faster autofocus.

However, there was still a need to create a high-powered AF motor for fast aperture lenses with larger focusing groups, which could work efficiently and deliver fast, smooth and quiet autofocusing. The result was the EF 300mm f/2.8L USM lens, with a ring-type Ultra Sonic Motor (USM) 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.

A decade later, in 2012, a new type of focusing motor was introduced, STM, named after its use of stepper motors. This was developed with video particularly in mind because it enables very smooth, quiet focus changes.

In 2016 Canon introduced Nano USM focusing, which combines the speed of ring-type USM with the quietness and smoothness of STM focusing.

That makes four types of USM motor – the ring-type, Micro, Micro II and Nano types. Like all AF motors, they all aim to convert an electromagnetic force into a rotational force to drive the focusing elements in the lens. What is different about USM motors is that they use ultrasonic vibration energy which is converted into rotational force.

The ring-type USM motor is the most widely used AF motor in the Canon EF lens range. To be effective, a ring-type USM motor needs to meet certain requirements. It 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. It 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. It should be simple to manufacture, and should start and stop quickly to ensure the best focus response. It should also be as quiet as possible in use.

In addition to these features, ring-type motors are also highly efficient and have low power consumption to maximise the camera battery life. Being ring-shaped, they are ideal for fitting within the lens barrel. Their focusing speed is very controlled, and they are stable across a wide range of temperatures, from -30°C to +60°C.

The ring-type USM is actually very simple in operation. It is composed of a rotor and a stator – an elastic body with a piezoelectric ceramic voltage element attached to it. Applying an AC current with a resonant frequency around 30kHz to the stator creates vibrations that cause the rotor to rotate continuously. The frequency of 30kHz is in the ultrasonic range, and this is where the USM motors derive their name.

The piezoelectric element generates ultrasonic waves which, a bit like ocean waves propelling a surfer, cause the rotor to create a rotational force that moves the focusing group. Switching the current between two different phases changes the direction of the ultrasonic waves. Consequently, the focusing group can be made to move in different directions, giving control over the direction, speed and degree of focus adjustment.

Unlike the ring USM, where the stator and rotor are separate parts, in the Micro USM design the rotor, stator and drive gear are combined into one unit roughly half the weight of a ring-type USM motor. While the more powerful ring-type USM is designed to fit in the circular barrel of a lens, making it ideal for use in large professional zoom lenses, the Micro USM motor was created to fit in a wide range of lenses without being restricted to the size of the lens barrel. Micro motors are also cheaper to produce, making them more suitable for use in consumer lenses where cost is an issue.

In principle, the Micro USM works in a similar way to a ring-type USM, with ultrasonic vibrations created by piezoelectric elements. There are four piezoelectric layers, each constructed from two alternating phase piezoelectric elements. These elements are offset from each other in alternating phases by 90°. Applying an AC current to only the A-phase causes the stator to 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.

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 reconfiguring the rotor and stator so that, instead of the two being aligned in a row, part of the stator is placed inside the rotor. This required the creation of a new format of vibration, so that the resonant frequency of the piezoelectric elements was not too high, resulting in insufficient vibrational amplitude.

The outcome 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. Its small size means the Micro USM II is well suited to use in compact zoom lenses. The first lens to feature the Micro USM II was the EF 28-105mm f/4-5.6 USM, which was launched in 2000.

The next focusing motor technology developed was a little different. First introduced in 2012, STM lenses are good for stills but they're great for video because the STM (stepper) motor produces smooth, quiet focusing movement.

A stepper motor uses DC (direct current) passing through multiple coils organised into groups. Supplying current to the groups in a sequence rotates the motor one step at a time. More groups enable more precise steps or movements to be made.

When compact size is paramount, Canon uses gear type STM technology. This uses helical gears to drive the focus without taking up much space. Larger lenses use a lead-screw type STM system. This is bigger than gear type STM units but it's faster and quieter.

The latest development, Nano USM technology was introduced with the updated EF-S 18-135mm f/3.5-5.6 IS USM lens in 2016. The aim was to produce a motor that can deliver the speed photographers want for stills with the smooth, steady adjustment required for video.

Like earlier USM motors, Nano USM uses ultrasonic vibration to create movement, but it's very small and still delivers high autofocus performance.

Like other USM units, the Nano USM motor has an elastic metal body, a ceramic voltage element and a drive unit. Sending current and varying the voltage applied to the ceramic elements creates two types of vibrations, which enable the motor to precisely control the speed and direction of the drive unit. However, the movement is linear rather than rotational – the lens focus elements are driven by a rack, with guide bars to control the forward and backwards movement. The outcome is smooth focusing with fine control over speed and near-silent operation.

Launched in October 2019, the RF 70-200mm F2.8L IS USM is the first lens to feature Dual Nano USM technology. It has two Nano USM motors, each driving different lens groups, working together to produce faster, more efficient focusing.