An optical instrument is provided for forming a viewable image of an object. The optical instrument includes an objective lens for forming the viewable image at an image plane, an eyepiece through which the user can view the viewable image, an eye sensor for sensing the direction of gaze of a user, and means for controlling a function of the optical instrument depending on the gaze direction. The eye sensor includes a light source for outputting light through the eyepiece towards the user's eye, a lens for causing the light from the source to be collimated when it exits the eyepiece, an imaging transducer for imaging the user's eye through the eyepiece and for generating an electrical image signal, and processing means for processing the image signal to determine the location of the user's pupil center and the location of a reflection of the light source to determine the gaze direction information.