An in-lens drive motor can be fine-tuned to correspond to specific lens characteristics. Canon was the first in the world to successfully employ an Ultrasonic Motor (USM) as an in-lens motor. Driven by ultrasonic oscillation energy, the ring-type USM is virtually silent, requires very little energy and is highly responsive to operational commands, making it an ideal AF motor. Initially incorporated in the EF300mm f/2.8L in 1987, the quiet, high-speed autofocus performance of the in-lens USM was hailed as an astonishing achievement. Mass production technology was established in 1990, and in the intervening years Canon has continued to develop sophisticated motors with enhanced performance, including ultra miniature and mass-market models.
And to enable photographers to convey their aesthetic more effectively, Canon developed a full-time manual focus feature that integrates autofocusing and manual focus capabilities. Once focus is achieved in AF mode, the photographer can fine-tune the focus with a touch of the focus ring to achieve a specific effect. A breakthrough feature for professional photographers, full-time autofocus is incorporated in all L-series lenses equipped with a ring-type USM.