An optical instrument comprising a housing having an optical path and a lens turret carrying at least two different optical components. At least three supporting devices are carried by the housing and engage the outer cylindrical wall of the lens turret at circumferentially spaced positions to support the turret for rotation about the longitudinal axis of the outer cylindrical wall to position a selected optical component in the optical path. Rotation of the turret is effected by a motor with a rotatable output pulley and a flexible drive element passing around the pulley and the outer cylindrical wall of the turret. One of the optical components is a rearwardly facing mirror, and the turret has a relatively large open area to enable light to pass therethrough for reflection by a mirror on the housing to the rearwardly facing mirror.