Telephoto lens based on automatic ranging, and method and device for realizing automatic focusing

The invention discloses a method for realizing automatic focusing of a telephoto lens based on automatic ranging. The method comprises the following steps of obtaining a distance value and theoreticalvalue of a clear focusing position of a motor through an object distance test experiment; forming a matrix type data list and generating a target code; carrying out power-on self-test, sampling for many times, and computing an offset value; carrying out lookup in a theoretical value information program according to the object distance change; and determining whether the fixed offset value is in apositive direction or a reverse direction based on comparison of an actual value of the focusing position with the theoretical value. The invention further discloses a device for realizing automaticfocusing of the telephoto lens based on automatic ranging. The device comprises a telephoto lens group, a laser ranging module, the motor, a motor control module, a pre-storage processing module, a main control module and a ser