As Canon has begun focusing more adamantly on making its DSLRs capable video capture machines, the company has had to adjust its glass to match. The new Dual Pixel AF on the 70D is a great improvement (check out these videos if you doubt that) but if you’re planning on shooting video using the autofocus, you’ll probably get to the point where you have to choose between buying a USM lens and an STM lens.
When that time comes, the video above by YouTube user marconilanna should make that choice a little easier by showing you the differences — in both noise and speed — between one of Canon’s STM lenses and a few older USMs.
First, the technical bit. USM stands for Ultrasonic Motor, while STM stands for Stepper Motor. Both are autofocus motors built into different lenses you’ll find in Canon’s lineup. General opinion seems to be that USM lenses are more professional-grade and better for shooting photos, while STM lenses are geared more towards amateurs and better for video, but why?
Take a look at the video at the top and you’ll find out. While USM lenses are lauded as being “almost silent” on Canon’s website, you can still pick up the focusing sound on the camera’s microphone when shooting video. As one Redditor put it “USM lenses sound like they’re possessed by a rodent.”
By comparison, the STM lenses really are almost silent, as they were built with video in mind — rodents need not apply. Their focus speed, while a tiny bit slower, is also better suited for video as it’s smoother and less “jarring.”
Finally, one of the big differentiators is that the STM lenses use a focus-by-wire system, which means that manual focusing will still make noise (albeit not much) and be slightly delayed since you’re not directly moving the focusing element when you turn the focus ring.
For most people, all of this adds up to the same “general opinion” mentioned above. If you’re an amateur who shoots a lot of video without manually pulling focus, you’ll probably go with an STM lens and be very happy. However, if you’re mostly shooting stills, the USM lenses will offer a faster focus with only a little bit of noise that shouldn’t matter even in quiet scenarios.
Of course, it’s up to you to draw your own conclusions once you watch the video above. Keep in mind, it’s not perfect: as one Redditor pointed out, the 40mm f/2.8 STM used in the video has a greater depth of field wide open than the 50mm f/1.4 USM, meaning it needed to focus a lot less. Still, the video gives you a good starting point if you’ve never seen the two technologies compared side-by-side.
P.S. We’re sure there are other practical differences between USM and STM lenses that we didn’t get a chance to mention in the post. If anything comes to mind, feel free to drop some knowledge in the comments and help out your fellow photographers.