If a person  can read letters at 20 feet that are readable at that distance to all people with normal vision, that person's visual acuity, or "eyesight", is said to be 20/20. If, on the other hand, a person is barely able to read letters at 20 feet that are readable at 40 feet to all people with normal vision.

Astigmatism
Astigmatism is a condition characterized by "blemishid vision"; that is, instead of light rays striking the retina in sharp focus, they spread out over a more or less diffuse area. Astigmatism is caused by the unequal curvature of either the cornea or the lens, sometimes both. Because the cornea and lens, are never optically perfect, most of us experience some degree of astigmatism. To correct astigmatism, the ophthalmologist prescribes glasses that compensate for the faulty curvatures of the refracting surfabes

Myopia
Myopi is an eye condition in which the focus falls in front of the retina. To remedy the defect, the ophthalmologis prescribes glasses with concave lenses that diverge the light rays hitting the cornea just enough to force the focus back on the retina.

Hyperopia
In contrast to the myopic eye, the hyperopic eye sees objects most clearly at far range. Thus instead of the focus falling in front of the retina. The usual cause of hyperopia is a flattened lens or cornea or an eyeball that is too short. To correct the condition' the refractive power of the eye must be increased by a convex lens, which cause parallel light rays to converge before entering the eye and, therefore, to come to a focus sooner.

Color Blindness
Color blindness is the inability to distinguish differences in color: it is generally partial and is seldom, if ever, complete.

Cataract
A cataract is an opacity of the lens of the eye. The so-called developmental cataract seen in young persons result from infection of the mother in the first trimester of pregnancy, or they may be hereditary. Degenerative cataracts, on the other hand, occur in elderly persons ("senile cataracts") and in persons exposed to excessive radiation and heat. Degenerative cataract make up the bulk of all cases. When there is considerable impairment of vision, operative removal of the lens is essential