An optical instrument comprising a focusing lens provided inside a lens barrel, a ring member that is arranged to rotate with respect to a lens barrel, and to slide to a first position and a second position, a focusing mode setting section, provided on a camera body, for setting focusing mode, and a control section for, when the ring member has been slid to the first position, carrying out focusing in a focusing mode that was set in the focusing mode setting section, and when the ring member has been slid to the second position, stopping focusing the focusing mode that was set in the focusing mode setting section, and performing focusing of the focusing lens to a distance corresponding to an absolute position of the ring member.