How the Eye Works
The eye is structurally similar to and functions much like a video camera. Light enters the eye through the cornea, the lens and an aperture known as the iris. As the different beams of light enter the eye, the light beams converge, coming into focus, and touch the surface of the retina. As the light touches the retina it sets off a chemical reaction creating an electrical signal that travels up the optic nerve to the brain. The brain then translates the electrical signals into a video image.
Sometimes the shape of the eye or defects in the cornea prevent the light beams from converging properly, resulting in a refractive disorder. There are four major types of refractive disorders are myopia, hyperopia, astigmatism and presbyopia.
When refractive disorders occur, special lenses are prescribed to correct the abnormality.
Understanding Corrective Lenses
Corrective lenses are similar to prisms. As light passes through the prism, it gets refracted, or bent, toward the thickest part of the prism. Corrective lenses are similar to two prisms, placed back to back. This configuration allows the lens to refract light in two directions, affecting the length of the beam as it enters the eye, and allowing the beams to converge on the retina.
Corrective lenses are designed to augment the eye’s natural function, which is why most corrective lenses are custom made. Lenses are made through a complex process of cutting, grinding and bending and each lens is specially made to correct the vision abnormality, in each eye, while preserving the preexisting functional vision in each eye.
Correcting Myopia (Nearsightedness)
With myopia, the eye is unable to focus on distant objects. As light enters the eye, the light beams from distant objects converge closer to the front of the eye causing the beams to cross. The crossed beams touch the retina, and the distant image appears blurry.
Lenses that correct myopia are thicker on the outer edges and thinner in the center. As light enters the lens it is refracted outward, extending the length of the beam, so that it converges on the retina, instead of at the front of the eye.
Correcting Hyperopia (Farsightedness)
With hyperopia, the eye is unable to focus on near objects. As light enters the eye, the beams from near objects do not converge at all before touching the retina—resulting in blurred near vision.
Lenses that correct hyperopia are thicker in the center, and thinner on the outer edges. As light enters the lens it is refracted inward, which shortens the beam, allowing it to converge on the retina.
Correcting Astigmatism
With astigmatism, abnormalities in the shape of the cornea or lens refract the light into two points of convergence, causing blurry vision at any distance. Astigmatism usually occurs in conjunction with myopia or hyperopia.
Lenses that correct astigmatism compensate for the abnormal shape of the eye’s structures, allowing light to refract into one point of convergence.
Correcting Presbyopia
Presbyopia is an age-related disorder where the cornea loses elasticity and is no longer able to stretch to and change shape to direct light to the retina. With presbyopia vision may be blurry at any distance.
Lenses that correct presbyopia tend to come in “bifocal” form to correct both near and distant sight. Bifocals lenses are split into upper and lower sections. The upper section corrects myopia, the lower corrects hyperopia.