Step Up
Q. How do lens stepping motors work?
A. Canon recently unveiled lenses with stepping motors optimized for video. Identified as STM lenses, they’re well-suited to the contrast-detection technology used for autofocus in video modes, which requires continuous sampling of the image sensor. With each sampling, stepping motors nudge lens elements toward sharpness. (Conversely, phase-detection AF—used for still capture—drives lens elements after a single reading.) Stepping motors can deal with the continual stopping and starting of contrast-detection AF more resiliently, quickly, and quietly than conventional AF motors, whose design causes them to struggle with the task. You can hear this struggle when a lens emits a stuttering sound as it tries to autofocus in live view.
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