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