Nearsightedness, medically known as myopia, is the most common cause of poor eyesight, especially in younger adults. It refers to being able to see well at close range, but not at a distance. It is the opposite of farsightedness (hyperopia). Focusing ability can start to decline in childhood and continue to slide through the teenage years, usually stabilizing in the early 20s. After that, most people find they don't need to change their glasses or contact lens prescriptions very often, if at all.
Myopia affects about 40% of persons aged 12 to 54 in the United States. We know that some ethnic groups are more prone than others. People of Chinese, Japanese, and Southeast Asian descent are especially likely to be myopic.
Myopia is caused by an eye whose shape doesn't permit it to focus on distant objects. 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 myopia, either the cornea is too curved or the eye is too long front to back, so the light rays meet and cross in front of the retina. By the time the image reaches the retina, it's blurred.
Nearsightedness is at least partly genetic. It isn't a disease. Just as some people's noses are longer than others, so are some people's eyes. Some research suggests that people who do a lot of work requiring that they focus closely may be more likely to develop nearsightedness over time. Children who are born prematurely often develop eye conditions that affect the shape of the eye and may be more likely to develop nearsightedness.