Canon made a splash earlier this month by announcing its first EF pancake lens, the Canon 40mm f/2.8 STM. If you’re considering this lens, one thing you should know is that the autofocus noise may interfere with your videos unless you use an external mic. In his review of the lens, photographer Dan Carr writes,
Here then is probably the biggest problem with this lens. With any other Canon lens, if you think the AF motor is making too much noise you can either switch to manual focus mode to disengage the focus motors or with Canons l-series lenses and their ultrasonic motors you simply just turn the focus ring manually yourself and it doesn’t engage the noise producing AF motor. Unfortunately though, the STM motor works in a different way […] Even when you switch to manual focus mode, rotating the the focus ring engages the STM motor to move the lens elements as the whole thing is a focus by wire system. This means that there is absolutely no way for you to get a silent video. Whether you let the camera do the focusing, as with the new cameras like the 650D/T4i , or whether you do it yourself, you are going to get the background hum as demonstrated in my video
It’s an interesting quirk, since the STM technology is meant to provide smooth and quiet focus for video recording. It may be quiet (here’s a comparison with the 50mm f/1.8 II), but you can’t eliminate it completely. On the flip side, the lens is attracting rave reviews.