STM
Leadscrew-type STM
Canon first used the Stepping Motor (STM) in its EF40mm f/2.8 STM lens. The STM has a smaller actuator than traditional ultrasonic motors. This was a significant factor in the creation of Canon's first pancake-style EF lens. The STM's simple mechanical structure ensures high responsiveness and controllability when starting and stopping. STMs are classified into two types: leadscrew-type STMs, which provide exceptionally smooth AF drive, and gear-type STMs, which are compact. The gear-type STM was used in the EF40mm f/2.8 STM lens, and it is still used in a number of RF-mount lenses. However, the RF10-20mm f/4 L IS STM was the first L lens to use STM, and with good reason.
The USM is better for lenses with long drive strokes, but the RF10-20mm f/4 L IS STM was designed for a shorter stroke. As a result, developers determined that an STM could be used while still providing satisfactory performance. The smaller actuator (compared to the USM) allows for more flexibility in designing the layout of the IS lens group. It is possible to mount the image stabiliser (IS) mechanism on the sensor side, reducing peripheral blur. Furthermore, the RF10-20mm f/4 L IS STM is the first RF lens with an STM that includes a position sensor. By reducing the start-up time, the lens's overall performance is comparable to that of the USM.
Gear-type STM