When corrective lenses are needed for a person to achieve clear vision, the condition should not be thought of as a disease. “Refraction” is the term that refers to the bending of light as it passes through one object to another. “Refractive errors” occur when the shape of the eye prevents light from focusing directly on the retina. The length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens can cause refractive errors. This helps explain how the common refractive errors of myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia (which makes reading glasses necessary) are related to the eyes’ refraction ability and not at all to disease-related conditions. During a refraction test, the doctor puts an instrument called a phoropter in front of your eyes and shows you a series of lens choices. He or she will then ask you which of the two lenses in each choice looks clearer. Blurred vision is the most common symptom of a refractive error.