A person who is nearsighted has difficulty seeing objects in the distance, although he or she can see close objects well. Nearsightedness is also called myopia.
In some cases, nearsightedness is an inherited condition caused by an abnormally long eye, as measured from front to back. Because there is a longer distance between the cornea (the clear "window" that covers the front of the eye) and the retina (the light-sensitive layer at the back of the eye), images tend to focus in front of the retina, rather than on the retina itself.
In other cases, nearsightedness is the result of a mismatch between the length of the eye and the ability of the eye's lens to focus an image in the correct location. Again, this causes images to focus in front of the retina, resulting in nearsightedness.
Currently, nearsightedness is the most common form of vision problem in the United States, affecting an estimated 25% of Americans. In many cases, genetic factors play a role in the condition. Several generations of the same family can have the problem.