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The Definition of Refractive Errors

The eye is a sophisticated light-gathering device. With all of it´s complexity, however, it works very much like a camera. Incoming light is focused (or refracted)by a two-lens system: first the cornea (clear covering over the front of the eye)and then the crystalline lens (located behind the pupil).

The inside of the eye is lined with a tissue called the retina. The retina contains light-receiving cells called rods and cones. Thus the retina corresponds to the film in a camera. The rods and cones gather incoming light and send the impulses to the brain through the optic nerve.You might think that the goal would be to focus light on the optic nerve. This is not the case, however. The optic nerve actually contains no rods or cones, and is thus a blind spot. Instead, the eye is designed to focus light on a special part of the retina called the macula. The macula consists almost entirely of cone cells, which are responsible for fine, central vision.In a normal, healthy eye, the incoming light is focused on the macula. However, every eye is not perfect. The image might be focused in front of or behind the macula. In this case the person’s vision is blurred and out of focus. We say that a refractive error exists.