One to two billion people see the world slightly out of focus
Refractive error means the eye’s optics fail to land a sharp image on the retina. Myopia, hyperopia, astigmatism, and age-related presbyopia blur distance, near vision, or stretch shapes. Roughly one to two billion people are affected—about a quarter of Europeans and four-fifths of Asians by some estimates.
A long eyeball over-bends light into myopia; a short one under-bends into hyperopia; an uneven cornea yields astigmatism; a stiffening lens brings presbyopia after midlife. Children usually start hyperopic and drift toward emmetropia or myopia as the globe lengthens. Family history raises odds. Symptoms can include double vision, headaches, and strain. Emmetropia focuses parallel rays from beyond about six metres without accommodation. Prescriptions report sphere (minus for near-sight, plus for far), cylinder for astigmatism amount, and axis in degrees 1–180 for its orientation. Unequal eyes (anisometropia) and unequal image sizes (aniseikonia) add further labels.
Glasses are the simplest, safest fix; contacts widen the field but carry infection risk; surgery reshapes the cornea or implants lenses. Concave lenses diverge rays for myopia; convex ones converge for hyperopia; reading glasses, bifocals, or progressives ease presbyopia. Adult myopia rates sit between about fifteen and forty-nine percent; childhood rates span roughly 1.2–42%. Far-sightedness clusters in young children and the elderly; presbyopia hits most people over thirty-five. Near-sightedness is the single most common refractive disorder worldwide.
In 2013 about 660 million people (ten per hundred) still lacked correction, and 9.5 million were blind from uncorrected refractive error—joining cataracts, macular degeneration, and vitamin A deficiency among leading vision-loss causes. Diagnosis rests on a clinical eye examination. The optics problem is often fixable with lenses or surgery; the access problem—getting glasses to those 660 million—is not always.
Source: Refractive error
