A method is described which allows the real diameter of an object on the fundus to be determined by means of a system of curves which are the result of trigonometrical calculations and which give an approximate value; only a photograph of the fundus, a measurement of the ametropia and of the radius of the anterior surface of the cornea are required. The results can be corrected with a second graph, which makes use of the length of the optical axis of the eye.