The Canon 40mm f2.8 lens uses an STM (stepper) motor that is supposed to provide much smoother focus when controlled by a camera like the t4i which utilizes full time auto focus in video mode. But how well does that motor perform with a USB follow focus controller?

For these test I’m using the Okii USB follow focus, but there are many other options on the market. Since there are several stepping options available on a USB follow focus controller I picked two of the settings that seemed to work the best for this lens. The first test looks a little bit jumpy and the movement is jagged, but when I made the switch to the second setting it was very easy to see that the STM motor provided a very smooth focus transition.

Because the Canon 40mm lens breaths a little bit during focusing (lens breathing explained), it’s sometimes hard to return to the original focus point with the same number of turns. Even so, the STM motor performed better then other lenses I’ve tested. This makes me optimistic about the hole STM motor system in general. If Canon starts to install this type of motor in nicer lenses, it could mean that USB follow focus systems would be a lot more useful in the future.

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