On many Canon lenses, you’ll either find STM or USM written which specifies the kind of motor that the lens uses for focusing. In this post, we’ll be talking about the difference between STM and USM and which is better for photography.
STM stands for stepping motor, which is a very smooth but slightly slower way of autofocusing.
USM stands for ultrasonic motor, which is faster and what Canon used to use in most of their lenses. These motors are not as smooth or quiet, though.
Where did STM and USM come from?
Originally, autofocus systems were operated using a motor that sat in the body of the camera rather than in the lens. A mechanical gear would connect the motor to the lens, and the motor inside the camera would rotate the lens to achieve a desired focus.
Back then, there were plenty of big camera manufacturers: Canon, Nikon, Yashica, and Pentax. Of these 4, Nikon, Yashica, and Pentax used a similar design to keep the motor in the camera body.
Canon designers decided to try another method by putting the motor in the lens. As it turns out, Canon’s designers hit the nail on the head and their system was far superior to the others. Eventually, everyone else switched to a system based on Canon’s(with their own tweaks, of course).
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How did we get to STM and USM lenses?
Canon lenses today use one of three kinds of motors: a STM motor, a USM motor, or a regular DC motor.
DC or direct drive motors
Even though most Canon lenses have STM or USM motors, if your lens does not specify it, it probably has a DC motor. a DC motor is the bottom of the pile in motors: there’s no smoothness of STM, no speed of USM, but it can do the job and as a budding photographer, it may not matter to you.
USM or ultrasonic motors
Ultrasonic motors are the most popular kind of autofocus motors and most Canon lenses use these. Ultrasonic vibrations are converted into rotational energy which is used to move the lens. For this to work, you need two loosely coupled rings. By vibrating one of the lenses, the other will rotate.
One ring is on the body, and the other ring is on the focusing part of the lens. USM motors have the advantage of speed. If you need to click really fast, the USM motor can keep up really well. You can also make small manual adjustments without having to turn autofocus off.
Ring USM motors
Even USM motors are of a few different types! Ring USM motors are the most common variation of autofocus motors in Canon lenses.
The motor itself is quite powerful and allows for a level of exact precision. You don’t need any mechanisms to reduce the speed of the motor. Additionally, once the motor has found a good position, you don’t need to keep applying power to it to maintain that position.
They are also very responsive, and it’s no surprise that Ring USM motors are the most common kind of motor found in Canon lenses.
Micro USM motors
Micro USM – as the name suggests – is a much smaller motor so it can fit into much smaller lenses. These are also cheaper so where micro USM motors are used, the price of the lens is less as well.
Micro USM uses a motor similar to the regular USM motor, but in this case, it’s not directly connected to the focusing rings. Instead, gears are used which actually makes it a hybrid between an old school autofocus system and the newer USM system.
Canon’s latest development is Nano USM, which is a high speed autofocus mechanism that is just as good as the regular USM and just as quiet and smooth. It’s probably the best kind of lens to use for everyday photography and even video.
You can shoot good, smooth video in which your camera focus keeps up with your moving subjects. Of course, it will shoot really good photographs, too.
STM or stepping motors
STM motors as you read above are smoother and quieter than USM motors. If you’re looking to shoot videos as well as photos, you can’t go wrong with a lens that uses STM motors.
Because it’s so quiet, your microphone won’t pick up any of the internal noises in the camera and you’ll be able to capture more of the actual subject’s sounds.
Stepping motors are called stepping(or stepper) motors because they can execute very tiny movements down to 0.1mm, and they can do them really fast. In fact, stepper motors are used in 3D printers for this very reason.
The small and precise movements are ideal for keeping up with the constantly changing focal length required when shooting videos.
This system is totally electronic and there are no gears or rings involved.
Smaller lenses using STM motors use gears to change the focus.
Lead-screw motors are used in larger lenses(where you have more space). Lead screw motors are even more quieter and can achieve faster speeds than gear STMs.
Which one should you get?
For professional applications and for shooting higher quality photographs, USM lenses remain the undoubted champion.
For amateur photographers who also shoot lots of video, STM lenses shine – they’re totally silent and can deliver really crisp video. The focus speed is a bit slower but that allows for more smoothness.