There are many different types and designs of conventional AF motors for photographic lenses, but they all operate on the principle of conversion of electromagnetic forces into torque. Ultrasonic motors, on the other hand, are based on an entirely different principle, when the rotation force is generated from the energy of ultrasonic vibration. Currently, ultrasonic motors are gradually replacing noisy and slower AF micromotors. Three types of ultrasonic motors are known, which can be classified by the method of converting of vibration energy into torque: a standing wave, a traveling wave and vibrating tongue.
For the first time in the world the ultrasonic motor was used in Canon EF lenses. Ring-type ultrasonic motor was introduced in 1987 and the first lens which used such type of motor became Canon EF 300mm F/2.8L USM. In 1990 Canon implemented the technology of mass production of ring-type ultrasonic motors for the popular lenses accessible to a wide range of photographers. A new type of ultrasonic motor — Micro USM — was introduced in 1992 which made possible automated production, and super-compact Micro USM II (half-sized in comparison with Micro USM) appeared in 2002. All Canon ultrasonic motors are based on the travelling-wave principle.
As for Nikon, its first lens with ultrasonic motor saw the light only 9 years later — it was Nikon AF-S Nikkor 300mm F/2.8D IF-ED supertelephoto lens. Nikon uses both ring-type and Micro USM (Compact SWM, according to Nikon’s terminology) type ultrasonic motors.
Advantages of ultrasonic motors:
- simplicity of design,
- good controllability and start/stop response,
- extremely quiet operation (virtually silent).
Additional advantages of ring-type ultrasonic motors include:
- high efficiency and low power consumption,
- circular shape of the motor is optimal for integration into the lens barrel,
- the possibility of high-precision and high-speed focusing,
- stable operation under severe conditions and in a wide temperature range.
As for the Micro USM / Compact SWM drive, these are multi-purpose miniature ultrasonic motors in which the force is transmitted via the gears, as in a conventional AF motor. Micro USM / Compact SWM drives have the following advantages:
- can be integrated into a variety of lenses regardless of their optical structures, as they are not limited by the diameter of the lens,
- have a more compact dimensions and lighter weight compared to a ring-type ultrasonic motor,
- the cost of production is lower which allows to integrate such motors in popular lenses.
In 2016 along with the introduction of the Canon EOS 80D camera, the company also introduced updated version of EF-S 18-135mm F/3.5-5.6 IS USM lens which features completely new type of ultrasonic motor, called Nano USM.
The need to combine speed for still image AF with smooth response and drive for video recording has been a challenge for Canon’s engineers until now, because in most situations, what constitutes good AF performance in a lens differs significantly for still image shooting and for video:
- For still images, users normally want fast AF speed that can start and stop almost instantly and accurately put the sharpest focus where the user wanted it;
- For video recording, users normally want smooth consistent AF speed without sudden starts and stops.
AF motors originally designed for superior AF performance during still image shooting — like Canon’s highly-regarded ring-type Ultrasonic Motors — tended to have less than ideal smoothness when asked to perform video-oriented AF. Despite the fact that STM motors are good enought to cope both with still images and video recording, the users now demand for an even higher performance during video recording.
In order to meet the new requirements, Canon engineers implemented a slightly different approach in developing of the new AF motor — using the same principles of ultrasonic vibration converted into movement, they developed a direct-drive system that is incredibly compact, easily placed within the body of a lens and has tremendous AF performance. Nano USM gets its name from the incredibly compact size of this motor. AF speed during still-image shooting rivals the high-end lenses with Canon’s powerful ring-type USMs — it’s nearly instantaneous. It’s a clear step forward from what we have come to expect from the consumer lenses based on Micro USM motors or the recent STM motors. Nano USM is also outstanding in video applications — AF operation has tremendous smoothness and is free from the start-stop focus movement. Another huge factor in video shooting is that the AF operation is nearly silent — very important if a microphone is attached to the camera and vital if using the camera’s built-in microphone. Nano USM matches the relative silence of previous STM designs during video recording. Due to it’s combination of focus characteristics, Canon’s Nano USM is an interesting and compelling possibility for many Canon EOS photographers.
At the moment, however, Nano USM is optimized for lenses with small and lightweight focusing groups. Lenses with large and heavy focusing elements are better served with more powerful ring-type ultrasonic motors.
The manufacturers of photographic lenses use the following designations to indicate that the lens is equipped with ultrasonic AF motor:
|Nikon||SWM||Silent Wave Motor|
|Pentax||SDM||Sonic Direct Drive Motor|
|Samsung||SSA||Super Sonic Actuator|
|Minolta, Sony||SSM||Super Sonic Wave Motor|
|Tamron||USD||Ultrasonic Silent Drive|
Unfortunately the manufacturers do not indicate the type of built-in ultrasonic motor. With a fairly high degree of accuracy the type of the motor can be determined by the availability of Full-Time Manual focusing — if the lens supports it, it’s a sign of ring-type ultrasonic motor. In case of Canon EF/EF-S lenses the only exception is Canon EF 50mm F/1.4 USM which has Micro USM but also supports Full-Time Manual focusing through the use of special mechanical coupling.
The accuracy of autofocus does not depend on type of autofocus drive (micromotor, stepping, linear or ultrasonic motor) but depends on focusing method (contrast or phase detection AF), efficiency of AF algorithms, lighting conditions and the number of other factors.