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The Canon EF 40mm f/2.8 has a designation of STM on the lens. What does this mean? What are the advantages of having it and does it replace an older technology?

We have a terminology thread that usually covers these questions but this is not yet addressed in it.

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I was just heading over here to ask the same thing myself! Great minds think alike... :) – Mark Whitaker Jun 7 '12 at 20:46
I mostly shoot video, will the Sigma 35 1.8 HSM be as quiet as the STM kit lenses (or quiet enough for video?) I would rather get the Sigma and be able to shoot wide open rather than limited by higher apertures with the STM kit lens. – user21299 Jul 26 '13 at 19:59
@damon James - that would fit better as a new question then a comment on this question. – dpollitt Jul 26 '13 at 21:57
@user21907 You're sales person doesn't understand what they're talking about. (Auto) focus in movie mode is a function of the body, not of the lens. STM lenses are better at it, but all my lenses will auto focus in movie mode. – Philip Kendall Aug 30 '13 at 13:19
@PhilipKendall See this quesiton. When the STM lenses were first announced, the announcement seemed to say that hybrid AF would only work in video mode with STM lenses. This turned out to not be true, but I can see a salesperson being initially confused (or, still confused). – mattdm Nov 1 '13 at 16:23
up vote 45 down vote accepted

STM stands for Stepper Motor and is a applied to a new range of Canon lenses which feature a new design of focus motors which, along with a new iris mechanism are designed to eliminate (auditory) noise during video recording.

Canon haven't revealed any information about how the new design works but it's probably the same type of motor used in mirrorless camera lenses. It's a more precise version of a regular DC motor but still has the same direct connection to the lens focus group, which means manual focus has to be implemented using a focus-by-wire arrangement whereby moving the focus ring by hand sends a signal to the motor to move the focus group.

In comparison an ultrasonic motor (like Canon's USM) consists of a pair of concentric rings which vibrate at high frequency to rotate back and forth, an arrangement which permits the user to move the focus ring to directly move the lens element, achieving full time manual focus without damaging the motor.

Stepper motors are better at producing smooth, precise incremental movements, such as those required by contrast detect AF, and AF during video. Ultrasonic motors are better at jumping to the right focus point as part of a phase detection system. See What is the practical difference between phase-detect and contrast-based autofocus?

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Stepper motors are a specific type of AC servo motor, As such, they're really not a DC motor. Additionally, DC motors can be made as or more precise then a stepper. The functional precision of a servo system is merely a product of the specific implementation, so saying a stepper motor is a "more precise" version of a DC motor is an over-generalization, in addition to being incorrect about the DC part. – Fake Name Jul 27 '13 at 9:32
Stepper motors rotate in fixed increments (steps) say 1/8 of a turn or 1/16 of a turn in response to a pulse of current. Steppers can be driven very precisely, in a digital fashion. – Kaushik Ghose Nov 2 '13 at 2:28
@FakeName, Why would you think a stepper motor is AC? And why would you think cameras have DC-to-AC inverters? And why are you colluding servos with motors? Stepper motors ARE more precise motors. Typical motors only control direction and power of rotation. Stepper motors drive to a specific radial output (the increments and therefore precision can vary). BOTH of these must be coupled with a sensor feedback loop for dynamic control. The sensors are the same. The control circuits aren't fundamentally different. ONLY the motor is different. And it's a pretty massive effect. – Travis Oct 3 '14 at 21:45
@Travis - 1. ALL motors are AC. "DC" motors just use brushes internally to convert the DC power to AC. Stepper motors however do not have brushes, so you have to externally commutate them. Colloquially, this type of drive is referred to as "AC", since it requires driving current in both directions through the field coils (which is basically AC). 2. I'm using the engineering definition of servo motor. 3. Precision is a function of the complete control loop. The motor type is irrelevant as long as the control loop can compensate properly. – Fake Name Oct 3 '14 at 22:06
TL;DR You can use stepper motors as a servo motor in a part of a larger servo system. The term "servo" motor has been polluted by RC model community to refer to one specific type of servo system that's commonly used for controlling models, but that's not the only type of servo motor, or servo system. – Fake Name Oct 3 '14 at 22:08

I don't know if Canon is using the term with their own twist, but in normal computer controlled motors, a stepper motor is quite a bit different than a normal electric motor. A stepper turns to one of a fixed number of positions (steps) and does not just turn "on" and spin. Rather, you step to a specific positions, say "clockwise 1/4 turn" and it goes exactly one quarter of a turn and stops. Not 5/16 and not 3/16. 1/4.

This means it is easy for the computer controller to say "go in 7/8 turn" and stay there.

I would not expect that a stepper is "cheaper", rather it is a different solution to a specific kind of problem.

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From what I could find, STM stands for (focus) stepping motor.

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I believe the point of the new focusing motor is to reduce vibration conducted through the camera body into the internal microphone. If you have a video-capable DSLR, you'll find that if you make it autofocus while taking video, you'll get a buzzing or rumbling in the audio track. – Warren Young Jun 7 '12 at 17:28
That sounds like a very likely explanation. – Håkon K. Olafsen Jun 7 '12 at 17:31
Dpreview announcements about the two latest canon lenses say the following "both models feature new stepper motor technology (STM)". So it definitely isn't about USM, seems more like a cheaper alternative optimized for better video recording and should work well with the eos 650D's phase detection AF. – Berzemus Jun 8 '12 at 7:46

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