The words “farsighted” and “nearsighted” are confusing to some people. It may help to remember that “sighted” is the opposite of blind. Thus, nearsighted people can see well for near objects, and farsighted people can see well for far objects. But there's more to it than that.
The cornea and lens together refract light to focus it on the retina. For perfectly refracting eyes, the lens is focused at “infinity” when the muscles inside the eye are at rest. When these muscles contract, they operate fibers attached to the lens to make the lens rounder, or more convex. This is the opposite of what I would expect if I had never read any eye reference books. It would be more straightforward for the muscles to pull the lens into a flatter shape, but that's not how it works. Nature has provided us with a mechanism to keep the eye muscles at rest most of the time. This is important because it doesn't work that way for farsighted eyes.
Nearsighted eyes are too long for the refractive power of their cornea/lens system, while farsighted eyes are too short. Nearsighted people can compensate to some extent by moving closer to the objects in view. Farsighted people can use their eye muscles to make their natural lenses more convex all the time. But there is a price to pay for this.
For eyes with perfect refraction, the lenses are at their most convex when viewing objects at very small distances. The two eyes will be almost crosseyed when viewing something so close. The muscles on the outside of the eyes work in cooperation with the internal muscles that change lens shape, so that the pupils move closer to the nose as the lenses become more convex.
When typing, most people have to lift their little finger at the same time as the ring finger. Try it; put your fingers on the “asdf” and “jkl;” keys, and then type a “w” with your left hand and an “o” with the right hand. The eye muscles that change lens shape and point the eye work together in a similar way, especially for young children.
Farsighted babies often cross their eyes because they're trying to focus. This is called accommodative esotropia. If untreated, this condition can cause amblyopia (lazy eye), and loss of visual acuity. However, a simple pair of baby eyeglasses can correct the problem for many children. Be sure to ask your doctor whether your child's vision is normal.
Clyde K. Kitchen, M.D., “Fact and Fiction of Healthy Vision: Eye Care for Adults and Children”, Praeger, 2007.
Linda Fugate is a scientist and writer in Austin, Texas. She has a Ph.D. in Physics and an M.S. in Macromolecular Science and Engineering. Her background includes academic and industrial research in materials science. She currently writes song lyrics and health articles.
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