The Sound of Movie Servo AF and the Importance of STM Lenses

While tinkering with the 7D Mark II and its Movie Servo AF feature, I wondered how much of a difference there would be in AF sounds captured by the built-in microphone when using various lenses. So I ran a small test...

I set up a the 7D Mark II on a slider so there would be a Movie Servo AF focus shift from a close subject (my Rokinon 8mm f/3.5 Fisheye) to a distant one (a fence) and tried the following lenses:

• Canon EF 85mm f/1.2L II USM
• Sigma 50mm f/1.4 DG HSM Art
• Canon EF 17-40mm f/4L USM
• Canon EF 40mm f/2.8 STM

I moved the slider setup with each lens to get approximately the same framing. Note that ambient traffic was a variable throughout the testing; it may have reduced the apparent AF noise in a couple of the tests. But overall, it was still easy to draw a solid conclusion from the tests.

What I noticed...

The AF sound was definitely audible in three out of the four lenses, while the fourth (the STM lens) was virtually silent. Coincidentally, there was almost no nearby traffic sound when recording the test with the EF 40mm f/2.8 STM, which makes its silence all the more impressive. In addition to being inaudible, the STM lens's focus transition looked much smoother and more natural because the 7D II can customize Movie Servo AF with STM lenses; mine was set to Movie AF Servo Speed (Slow 3).

That's a big reason why the EF-S 24mm f/2.8 STM will be an excellent investment for anyone who has a 7D Mark II (or 70D, for that matter) and wants to create high-quality videos with the convenience of AF. And even though externally recorded audio will always be best (I use a Tascam DR-07 & Zoom H2N), it's nice that you can fall back on the camera's built-in audio when using AF with STM lenses if anything should happen to your main mic(s). [Sean]