A lens assembly includes a base frame, an image sensor, a lens barrel, a lens and a backoff mechanism. By means of the backoff mechanism, the image sensor is mounted at the base frame and movable between an operative position and a non-operative backoff position. When the image sensor is in the operative position, the lens barrel is in a first position far frame the base frame, and the optical axis of the lens intersects the image sensor. When the lens barrel is moved to a second position close to the base frame, the image sensor is moved to the non-operative backoff position, and the optical axis of the lens is not intersected with the image sensor.