Binocular fusion with convergence: (diopters, prism convergence units) This test measures how well someone can adapt to challenges in focusing light on the retina. There are two almost identical tests. One test occurs with an object placed at 3m from the seated participant, and the other with an object at 30cm from the seated participant. Light from an image is passed through a prism. This is analogous to moving the image further away from the body. In response, the eyes must diverge (separate) to focus on the object, just as they would if the image actually moved away from the body. Different prisms are used to create increasing challenges. The score for these tests is simply the maximum amount of prism convergence (dioptres, noted on the prism, as one would note diopters on eye glasses) that the seated participant can accommodate at 3m and at 30cm.