According to the American Academy of Ophthalmology, refractive errors of the eye are the most common eye condition; however, they are not diseases. To have clear vision, the light that enters the eye has to be bent (refracted) through the cornea (the clear portion at the front of the eye) and the lens and come into focus on the retina (the layer of cells at the rear of the eye) for clear vision to occur. However, when the shape of the eye is not normal, refractive errors happen and vision becomes blurry. There are four main types of refractive eye problems: nearsightedness (myopia), farsightedness (hyperopia), astigmatism, and presbyopia.

Nearsightedness occurs when the eyeball itself is too long or the cornea is too pointed. This causes the light that enters the eye to come into focus in front of the retina instead of on the retina. People who are nearsighted can see objects that are close to the eye very clearly but items in the distance are blurry. This condition can get worse during the teenage years because this is a period of rapid growth. People who are very nearsighted may be at higher risk for developing a detached retina as explained in Eye surgery types article.

In a person with farsightedness, the eyeball is smaller than normal. Light focuses behind the retina in hyperopic eyes; objects that are close to the eye like the words in a book are blurry and difficult to see. This condition can be seen in children; parents may notice the child squinting or rubbing the eyes. Hyperopia is often an inherited condition; however, the condition may improve in children as they get older.