The cornea, conjunctiva, and intervening transition area known as the limbus comprise
the tissue at the ocular surface. The cornea functions as a protective membrane and a
“window” through which light rays pass to the retina.
In the ideal refractive state, emmetropia, an image is focused directly on the retina,
resulting in the perception of a sharp image in a healthy visual system. In myopia (nearsightedness),
the image appears focused anterior to the retina, producing a sharp image
at near and a blurred image at distance. The inverse is true in hypermetropia (farsightedness),
where the image is projected posterior to the retina, producing a blurred image
at near and a sharper image at distance. Astigmatism (cylindrical error) is the result of
two different refractive powers between two perpendicular meridians. As an object is
brought nearer to the eye, focus is maintained by an increase in the power of lens of
the eye; this is accommodation. The ability to accommodate decreases with age (presbyopia).
http://repub.eur.nl/res/pub/26785/111102_Saelens%2C%20Isabelle%20Elise%20Yannic.pdf
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