This test is used to determine whether you have squint (tropia) or phoria (latent deviation) and also measures the degree of deviation using a prism. It is usually done at far (6metres) and at near (40cm). The test requires that you focus on an object in front of you while the eye doctor covers each of your eyes alternately and observes the uncovered eye for movements. Abnormal movements usually indicate the presence of a squint and /or other binocular vision problem.