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The upcoming Canon 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 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

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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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From what I could find, STM stands for (focus) stepping motor. Not sure how that will affect the accuracy and speed. Maybe it's cheaper than USM, but better than what they had before?

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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
up vote 3 down vote

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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