The Hyper Sonic Motor (HSM) is an original Sigma development that uses ultrasonic waves to drive the autofocus mechanism. It’s extremely quiet operation helps avoid disturbing photographic subjects. High torque and speed assure rapid autofocus response. Sigma uses two types of HSM: ring HSM and micro HSM. The Ring HSM configuration permits manual fine tuning of focus (Manual Override) by turning the focusing ring after autofocus is complete.