In the emmetropic (normally refracted) eye, entering light rays are focused on the retina by the cornea and the lens, creating a sharp image that is transmitted to the brain. The lens is elastic, more so in younger people. During accommodation, the ciliary muscles adjust lens shape to properly focus images. Refractive errors are failure of the eye to focus images sharply on the retina, causing blurred vision (see figure Errors of refraction).
Errors of refractionIn myopia (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 (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 because of their ability to accommodate. To correct hyperopia, a convex (plus) lens is used.
In astigmatism, nonspherical (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. Cylindric 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.
Anisometropia is a significant difference between the refractive errors of the 2 eyes (usually > 3 diopters). When corrected with eyeglasses, a difference in image size (aniseikonia) is produced; it can lead to difficulties with fusion of the 2 differently sized images and even to suppression of one of the images.
Myopia and hyperopia are corrected with spherical lenses. Concave lenses are used to treat myopia; they are minus or divergent. Convex lenses are used to treat hyperopia; they are plus or convergent. Astigmatism is treated with cylindrical lenses. Corrective lens prescriptions have 3 numbers. The first number is the power (magnitude) of spherical correction required (minus for myopia; plus for hyperopia). The second number is the power of cylindrical correction required (plus or minus). The third number is the axis of the cylinder. As an example, a prescription for a person with myopic astigmatism may read -4.50 + 2.50 × 90, and a prescription for a person with hyperopic astigmatism may read +3.00 + 1.50 × 180.
The eye normally creates a clear image because the cornea and lens bend (refract) incoming light rays to focus them on the retina. The shape of the cornea is fixed, but the lens changes shape to focus on objects at various distances from the eye. By becoming more rounded, the lens allows near objects to be focused. By becoming flatter, the lens allows objects farther away to be focused. When the cornea and lens cannot focus the image of an object sharply on the retina, it is called a refractive error.
Did You Know...
The lens of the eye is flexible and changes its shape to focus on objects at various distances but begins to lose this flexibility after about age 43.
The lens and cornea may not bend light rays to focus them on the retina correctly for several reasons.
Nearsightedness (myopia) occurs when the eyeball is too long for the refractive power of the cornea and lens. Because of the corneal curvature and relatively long size, light is focused in front of (rather than directly on) the retina, and the person has trouble clearly seeing distant objects. In children, nearsightedness frequently increases until children stop growing.
Farsightedness (hyperopia) occurs in some people when the eyeball is too short for the refractive power of the cornea and lens. Because of the relatively short size, light is focused behind the retina. Children and young adults who are mildly farsighted may be able to see clearly if their lens is flexible enough to properly refocus light on the retina. However, with aging, the lens stiffens. Thus, as farsighted adults age, seeing near objects clearly becomes more difficult and seeing distant objects also becomes more difficult. Blurring of nearby objects is worse in dim light.