The major anatomical components of the eye are illustrated in Figure 1. From the point
of view of light entering the eye, the first refracting (light bending or focusing) structure
is the cornea. Approximately two-thirds of the focusing power of the eye is due to the cornea. After the cornea, light enters the anterior chamber and is partially blocked by
the pupil, which is the opening in the iris. Light entering through the iris is then focused
by the crystalline lens in the eye, which is the only structure in the eye with significantly variable power. The crystalline lens is focused by a muscle attached around the
periphery of the lens. The light then passes through the vitreous and focuses on
the retina (if properly corrected). If the light does not focus onto the retina, then an uncorrected refractive condition exists.

Refractive conditions are most commonly treated with corrective lenses -- either spectacle lenses or contact lenses. The most common ones are near-sightedness (myopia), far-sightedness (hyperopia), astigmatism (when the eye is shaped like a football instead of like a basketball), and presbyopia (a condition caused by aging of
the crystalline lens and leading to the need for reading glasses). Referring to Figure 2,
in a "perfect" eye light from a distant object focuses directly onto the retina when the eye
is relaxed or unfocused. In near-sightedness, distant light focuses in front of the retina, and requires a concave lens to bring the focus back onto the retina. In far-sightedness, distant light focuses behind the retina when the eye is relaxed, and requires a convex lens to bring the focus back onto the retina. In an astigmatic eye, the cornea or front surface of the eye is not spherical, and the power of the eye is different depending
upon the plane in which the incident light approaches. A lens with different curvatures
in different planes of rotation is required.

	Figure 1. Anatomy of the eye
Figure 2. The "perfect" eye	Near-sightedness	Far-sightedness