A lens drive system is the mechanical or electromechanical arrangement used in autofocus SLR and other camera systems to physically move the focusing elements within the lens in response to the camera's autofocus calculations, driving the lens to the correct focus position quickly, quietly, and accurately. The design and performance of the lens drive system has a direct and significant influence on the speed, accuracy, and operational noise of the autofocus system, making it an important consideration for photographers whose work demands fast, reliable, and unobtrusive focus acquisition.
Two principal types of lens drive system have been widely used in autofocus camera systems. The first places the autofocus motor within the camera body rather than in the lens itself, using a mechanical drive shaft or coupling that extends from the camera body and engages with a corresponding drive socket on the lens mount when the lens is attached. When autofocus is activated, the body motor drives the shaft, which in turn rotates the focusing helical within the lens and moves the focusing elements to the required position. This body motor drive approach allows simpler and less expensive lenses to be manufactured without their own focusing motors, as the drive mechanism is shared across all compatible lenses via the single motor in the camera body. However, the mechanical coupling between the body motor and the lens introduces some degree of noise, vibration, and potential wear at the drive interface, and the relatively long mechanical path from motor to focusing elements can limit the ultimate speed and precision of focus acquisition.
The second and now more widely adopted approach places a dedicated autofocus motor directly within each individual lens, eliminating the mechanical body to lens coupling entirely and allowing the motor to drive the focusing elements more directly, efficiently, and quietly. In-lens motor systems offer several significant advantages over body drive systems, including faster and more precise focus acquisition, greatly reduced operational noise - particularly important for video recording and wildlife photography where shutter and focus noise can disturb subjects or be picked up by microphones - and the ability to optimise the motor type and drive characteristics for the specific optical design and focusing requirements of each individual lens.
Various types of motor have been used in in-lens autofocus drive systems, each with its own performance characteristics. Conventional DC motors connected to the focusing mechanism via a gear train were among the earliest in-lens motor designs, offering adequate performance but generating noticeable noise from the gear mechanism. Ultrasonic or ring type motors - marketed under proprietary names such as Canon's USM, Nikon's Silent Wave Motor, Sony's Super Sonic Motor, and similar designations by other manufacturers - use piezoelectric actuators to generate ultrasonic vibrations that drive a ring gear silently and with great speed and precision, and have become the preferred drive technology for high performance professional and enthusiast lenses. Linear and stepping motor systems, increasingly common in mirrorless camera lenses, offer very smooth and near silent operation particularly well suited to continuous autofocus during video recording.