Farsightedness, known medically as hyperopia, refers to being able to see fairly well at a distance but not close up.
Hyperopia affects about 25% of the population.
The cornea is a special layer on the surface of the eye that does most of the work of bending light as it enters the eye. Light must be bent properly so that it creates a sharp image. When parallel light rays pass through the cornea, they should be bent just enough to meet and focus on the retina (the light-sensitive membrane that lines the back of the eye). The lens is responsible for fine adjustments of focus.
In farsightedness, the light rays meet at a point behind the retina. This causes nearby objects to be blurred. It could be said that the eye is too short back to front, or alternatively that the cornea is too flat.
Farsightedness is at least partly genetic. Many children are born with a degree of farsightedness, but some are able to counteract it with strong eye muscles. These muscles squeeze and stretch the lens just behind the cornea, which fine tunes the focus after the cornea does most of the work of bending light. In some children, the close vision becomes naturally sharper as they reach adulthood.
As people get older, their eye muscles become less able to compensate for hyperopia. After age 60, most people have some difficulty seeing close-up details clearly. This is called presbyopia. It becomes increasingly common in the years after age 50. The lens becomes thicker and more rigid, and the range of distances at which you can focus (called accommodation) narrows.