In a normal eye without refractive error (emmetropia), the light rays entering the eye are focused sharply by the cornea and lens onto the retina forming a clear image. When an eye fails to focus the images sharply on the retina, it is said to have refractive error. The poorly focused image causes blurred vision. Refractive error can be diagnosed with an eye examination and treated with corrective glasses, contact lenses or refractive surgery. There are different types of refractive errors. The commonest types are myopia, hyperopia, astigmatism and presbyopia. WHO estimates that over 150 million people worldwide live with visual impairment due uncorrected refractive errors making it one of the commonest eye disorders.
The three most common refractive errors are:
- Myopia (nearsightedness): difficulty in seeing distant objects clearly
- Hyperopia (farsightedness): difficulty in seeing close objects clearly
- Astigmatism: distorted vision resulting from an irregularly curved cornea, the clear covering of the eyeball.
In myopia, or nearsightedness, the point of focus is in front of the retina because the cornea is too steeply curved, the axial length of the eye is too long, or both. Distant objects are blurred, but near objects can be seen clearly. To correct myopia, a concave (minus) lens is used. Myopic refractive errors in children frequently increase until the child stops growing.
In hyperopia, or farsightedness, the point of focus is behind the retina because the cornea is too flatly curved, the axial length is too short, or both. In adults, both near and distant objects are blurred. Children and young adults with mild hyperopia may be able to see clearly due to their ability to accommodate. To correct hyperopia, a convex (plus) lens is used.
In astigmatism, non-spherical (variable) curvature of the cornea or lens causes light rays of different orientations (eg, vertical, oblique, horizontal) to focus at different points. To correct astigmatism, a cylindrical lens (a segment cut from a cylinder) is used. Cylindrical lenses have no refractive power along one axis and are concave or convex along the other axis.
Presbyopia is loss of the lens' ability to change shape to focus on near objects due to aging. Typically, presbyopia becomes noticeable by the time a person reaches the early or mid 40s. A convex (plus) lens is used for correction when viewing near objects. These lenses may be supplied as separate glasses or built into a lens as bifocals or variable focus lenses.