Canon Lens Class and Technology Abbreviations - Arc-Form Drive is the first autofocus motor used in Canon EF lenses. It is much louder than USM motors, somewhat slower and not as quick. It also has slower reaction time so does not follow subjects as well as newer motors. There is no full-time manual focus override. If an autofocus Canon lens has no indication on its barrel what sort of AF motor is used, it is either AFD or Micro Motor.