A rear focusing system is a lens design in which only the rear lens group moves to achieve focus, rather than the entire lens assembly. This approach eliminates changes in the physical length of the lens during focusing, making it lighter, faster, and more stable.
Rear focusing systems are especially advantageous in telephoto and zoom lenses, where precise and quick focusing is critical. By reducing the movement of lens elements, this system also helps maintain optical performance and image sharpness, providing photographers with a reliable tool for both still and action photography.