The Human Eye and Vision Defects

A natural lens system and how to correct it.

Light enters through the transparent cornea, passes the pupil, and is focused by the eye lens onto the retina. Muscles change the curvature of the lens so near and far objects can both be focused, a process called accommodation.

In short-sightedness distant objects focus in front of the retina and a concave lens corrects it. In long-sightedness near objects focus behind the retina and a convex lens corrects it. A normal eye has a near point of about 25 cm.

• The iris adjusts the pupil size to the light level.
• Myopia: concave lens. Hypermetropia: convex lens.
• Presbyopia comes with age as the lens stiffens.

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