Good vision depends on how well your cornea and lens focus light rays on the retina. Light rays must bend (refract) to focus on the retina. The retina is the light-sensitive nerve layer that lines the back of the eye. It creates impulses from the light rays that are sent through the optic nerve to the brain.

Refractive errors are vision problems caused mainly by a cornea that is not shaped perfectly or an eye that is longer or shorter than normal. The cornea is the clear part at the front of the eye. It bends and focuses light waves. Refractive errors cause light from an object not to be focused on the retina. This causes a blurred image. Refractive errors can occur in otherwise healthy eyes.