A binocular indirect opthalmoscope has a light source for producing an illumination beam and an illumination mirror (7) for directing the illumination beam to an eye to be viewed. The opthalmoscope also has, viewing optics including left-hand and right-hand eyepieces (12,13) and an optical splitter (9,10) for directing light along left-hand and right-hand viewing paths to the two eyepieces. An aperture holder (20) is pivotally mounted in a frame (3,5,6) of the opthalmoscope and adjusts the size of the illumination beam, under the control of a manually rotatable control knob (24). The movement of the aperture holder is transmitted, by means of mechanical linkage, to a moveable carriage (15) on which is mounted both the optical splitter and the illumination mirror, so that when the user moves the control knob to adjust the size of the illumination beam the optical splitter and the illumination mirror are also adjusted in position with respect to the opthalmoscope frame.