"About 4 percent of preschoolers have myopia (nearsightedness), 21 percent have hyperopia (farsightedness), and 10 percent have astigmatism (irregular curvature of the eye), according to a study funded by the National Institutes of Health," says a National Eye Institute's webpage. "The largest of its kind, the study identified related risk factors, including age, ethnicity, exposure to smoking, and access to health insurance. Uncorrected refractive errors, such as myopia, hyperopia, and astigmatism are the most common causes of vision impairment in children. Caused by slight irregularities in eye shape, refractive errors affect how light focuses on the retina, the part of the eye that converts light into signals the brain perceives as images. Refractive errors are correctible with eyeglasses, but uncorrected they can lead to lasting vision problems such as strabismus (misalignment of the eyes) and amblyopia (decreased vision in one or both eyes that cannot be corrected with eyeglasses)." Read more.