The STepping Motor (STM) is a type of AF motor that is able to control the rotational operation of the motor using the fluctuation of pulse signals. Each electrical pulse signal can rotate the stepping motor by one step, and start-stop response is superb. The simple mechanical structure makes it easy to adopt a compact lens design. Also, the STM helps to reduce operating sound, and is thus ideal for video capturing.
Canon developed two different types of STM according to the lens characteristics. The «STM + lead screw» unit adopts a lead-screw-type drive that is capable of generating a large torque, and enables silent and smooth AF on zoom lenses. The «STM + gear» unit allows for compact lens designs, and is optimised as AF motor for compact lenses (f.e pancake lenses).
Canon STM supports Full-Time Manual focusing: you have to half-press the shutter button, wait until the camera locked the focus automatically and only after that you may adjust the focus manually using the manual focusing ring (do not release the shutter button during manual focusing).
Canon’s first lens equipped with stepping motor was EF 40mm F/2.8 STM announced on June 8, 2012. As for the Nikon, their first lenses with stepping motor — AF-P Nikkor 18-55mm F/3.5-5.6G and AF-P Nikkor 18-55mm F/3.5-5.6G VR — debuted on January 5, 2016.
Lens manufacturers use the following designations to indicate that the lens is equipped with stepping AF motor:
|Nikon||AF-P||Auto Focus — Pulse Motor|
|Olympus||MSC||Movie and Still Compatible Linear Motor|
The accuracy of autofocus does not depend on type of autofocus drive (micromotor, stepping 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.