The optical and refractive aberrations

The three frequent refractive-aberrations of the eye are: far-sightedness or hyperopia, short-sightedness or myopia and astigmatism. When there is no refractive aberration the condition is called emmetropia.
The eye functions like a camera. Entering rays are first refracted by the exterior clear eye-layer, the cornea, then by the lens and finally focused on the retina in the back of the eye. The retina functions like the sensitive plate of a camera.
The iris or the coloured part of the eye functions like a shutter to regulate the amount of light entering the eye. In some cases, due to an abnormal curvature of the cornea or an abnormal distance between the cornea and the retina, the image is not focused on the retina but in front of it or behind it.

In a normal or emmetropic eye the curvature of the cornea is such that entering rays are focused on the retina, forming a clear image. This image is transmitted to the brain via the optic nerve. (fig.1)

In a short-sighted or myopic eye, the entering rays are focused in front of the retina and form there a blurred image. Without expedient, a myopic eye sees better at a short distance than a long one. The word myopia is derived from the Greek word "closed eye" or "slit eye", because people suffering from such a distortion tend to close their eyes a bit in order to see better at distance. Myopia usually appears between the ages of 10 and 12. Then the distortion progresses until the age of 18 to 20. Between 20 and 40 myopia changes only slightly or not at all. Reading with inadequate lighting, whether wearing glasses or not, too much reading or malnutrition, don't cause or influence myopia at all. As the tendency towards myopia is congenital, the evolution cannot be stopped.

The eye that is affected with hyperopia or farsightedness focuses the entering rays behind the retina. Without expedients a hyperopic eye sees better at a distance. (fig. 3c) Far-sightedness can increase with age.

The cornea of an astigmatic eye has an asymmetrical or irregular form. The entering rays are refracted in different ways and cannot form a clear image on the retina. Astigmatism, either combined with hyperopia or myopia or not, is usually congenital and changes only slightly during a lifetime. Small aberrations of astigmatism occur fairly frequently and don't have to be corrected.
In a normal eye the curvature of the cornea is as round as a football, whereas in an astigmatic eye the curvature of the cornea is oval like a rugby-ball.

Presbyopia or "reading-difficulties" occurs when the focusing-ability of the lens decreases. This is a normal infirmity of aging and usually appears at about 45.