What is a Refractive Error?

Refractive error is a term that is used to describe an inability of images to focus properly on the retina of the eye. The goal of glasses, contact lenses, and refractive surgery is to correct or improve these errors by helping images to focus closer to or onto the retina.

Myopia, often known as nearsightedness, occurs when the eye is too long or the cornea is curved too steeply. Myopia is usually the result of a larger than normal eye. In the myopic eye, light rays from distant objects focus before they reach the retina, causing distant objects to appear blurry.
Hyperopia, often known as farsightedness, occurs when the eye is too short or the cornea is too flat. As a result, a person can see distant objects more clearly than near objects. The images focus is directed beyond the retina and cause close objects to appear blurry.
Astigmatism occurs when the cornea is not round like a basketball, but curved more like a football. This uneven curvature prevents light rays entering the eye from focusing to a single point on the retina. Light focuses in two different locations in the astigmatic eye. The images focus at different planes and cause objects to appear blurry both at distance and near. Often people have astigmatism as well as myopia or hyperopia.
Presbyopia begins to occur in most people around age 40 when the lens of the eye begins to lose its elasticity (or flexibility). It is a normal part of aging of the eye which makes it difficult to focus on near objects and is usually corrected with bifocals or reading glasses. Presbyopia should not be confused with hyperopia.
Emmetropia is the state of the eye in which no refractive error is present, there is no need for glasses or contact lenses. In emmetropia, the curvature of the cornea, the shape of the lens and their distances from each other all work together to focus the rays of light to create a sharp image on the retina.