When we look at a photograph or movie, one of the first things that we notice is whether it is in focus or not.
Since 1987, when Canon became the first company to integrate a focusing motor inside the lens, mulitple motors have been introduced that are designed to acquire and track focus for fast moving subjects. All while retaining control for focus accuracy, smoothness and silence.
There are currently three main types of focusing motor technologies used within Canon lenses. These are STM, USM and a more conventional DC motor. Let’s take a look at the differences between these motors, so you can understand and select a lens according to your needs.
STM lenses shoot great photos and even better videos. They’re named after an element of technology called a Stepping Motor, which lets the lens focus smoothly and quietly – two great characteristics to have when capturing video.
Whilst some motors used in lenses make very audible and distracting mechanical noises when they focus, STM lenses focus quietly, allowing you to record more of what you naturally hear in an environment. Canon's STM technology provides a range of extremely quiet lenses that are still fast enough to capture photos in most scenarios.
Gear type STM
Our smaller, more compact range of STM lenses including the EF 50mm f/1.8 STM features ‘gear type’ STM – a very compact STM motor that uses helical gears to drive the focus.
Lead-screw type STM
The physically larger range of STM lenses features the ‘lead-screw type’ STM – which when compared to the ‘gear type’ offers even greater levels of silence and speed.
USM (Ultrasonic Motor) is the most widely used AF motor type in the Canon EF lens range to date. Common to all USM motors is the fact that that they convert ultrasonic vibration energy into rotational force to move the lens. This is the currently the fastest focusing motor in the Canon range, yet it allows you to still manually fine-tune focus without having to switch off autofocus completely.
Ring-type USM is found in most of Canon’s professional lenses – it allows for control and gives rapid speed and accuracy. The USM motor is powerful enough to drive heavy lens groups in large telephoto lenses quickly and easily, while still being able to avoid the need for a gear system to reduce the speed. USM motors also 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. The Ring-type USM motor is not as silent as the STM motors, but they are still extremely quiet for the performance they deliver.
The ring-type USM mechanism 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.