If your eyesight isn’t perfect, this test helps your doctor determine the right prescription for glasses or contact lenses. You look into a mask-like device called a phoropter, which holds lenses of various strengths. As you focus on an eye chart, your doctor will flip two lenses into your view and ask if the letters are more clear or less clear. By repeating this step with different combinations, the doctor can pinpoint the power that gives you the best possible vision. If you currently wear glasses or contacts, the doctor will check to see whether your prescription has changed.