How clearly you see depends in part on the shape of your cornea (the clear front window covering of the eye). The cornea and the lens work together to focus light into the eye.
For perfect vision, light rays must come into focus precisely on the retina. The retina is a light sensitive tissue at the back of the eye that works like film in a camera. If the light rays don't focus precisely on the retina, vision will be blurred or distorted. As a result, errors of refraction (nearsightedness, farsightedness, presbyopia, or astigmatism) may occur.
Nearsightedness also called Myopia. Myopia is caused when light rays entering the eye focus in front of the retina instead of directly on the retina.
Farsightedness is also called hyperopia. With hyperopia you can see things clearly far away, but things that are up close and blurry. In hyperoptic people the light rays entering the eye focus in back of the retina instead of directly on the retina.
Another type of focusing problem is astigmatism. In the case of astigmatism the cornea is oval shaped rather than round, causing both distant and near objects to look distorted. Astigmatism may coexist with either myopia or hyperopia or exist alone and is relatively common.
Another type of farsightedness that occurs around the age of 40, when the eye's focusing system weakens and is no longer strong enough to allow one to see comfortably at close distances is presbyopia. It is caused by a loss of flexibility of the lens inside the eye that occurs as a normal part of the aging process. Vision correction is then required with the aid of reading glasses or bifocals. Presbyopia can be treated with mono-vision.