Convergence fusional proximum: (diopters, prism convergence units) This test is similar to (2) above. When an object is moving closer to our head, the eyes symmetrically converge. When the object is moved beyond the participant's ability to converge, the participant will start to see two images (double vision). This test measures the distance between the bridge of the nose and point where double vision (cm) occurs in seated participants as an object is moved closer.