Abstract: A binocular optical apparatus provided with an objective and eyepiece lens system and using a mirror system for deflecting a light beam from the illuminated object to the eyepieces. The interocular separation, or distance between the eyepieces is adjustable by way of making the optical axis of at least one eyepiece swivelable relative to the optical axis of the other eyepiece at an angle determining the interocular distance of the eyepieces within an angle conforming to the range of natural eye movement such as to avoid eye strain. One of the mirrors of the mirror system is angularly movable at half the angle of motion of the movable eyepiece during adjustment to an appropriate interocular distance, by means of a lever drive arrangement, such that the image projected by the mirror into the eyepiece plane remains stable during adjustment of the interocular distance. The invention has particular applications for microfilm binocular viewers having a high magnification rate.