a lens assembly includes a third lens group frame having an end face extending in an optical - axis direction and provided with an end - face cam having displacement along a peripheral direction , for retaining a third group of lenses , a second lens group frame having a cam follower abutted against the end - face cam for retaining a second group of lenses , a second lens group spring for biasing the second lens group frame toward the third lens group frame , rotational regulator for regulating rotation of one of the third lens group frame and the second lens group frame around an optical axis , and a rotator for rotating the other of the third lens group frame and the second lens group frame around an optical axis when zoom operation is performed , wherein the end - face cam and a cam follower are operated by relative angular movement of the third lens group frame and the second lens group frame , around the optical axis at the time the zoom operation is performed to modify spacing therebetween in the optical - axis direction .