Review on Canon EF 40mm f/2.8 STM Lens - Fixed Black (6310B002) for US Cameras by Amar Singh ᠌

The main drawback is the focus mechanics. It differs from standard lenses in that there is no direct connection between the "focus" ring and the lens itself. The ring works like a joystick. You turn the ring, the lens gives a command to the camera, and it already moves the lens with a motor. Perhaps this helps in "silent" autofocus, but focusing manually is extremely difficult. The lens moves late, so it's hard to know if you've turned the knob correctly. Hence the constant failures. I do not recommend taking.