Like any other part of the body, the eye grows, works, tires, and ages. It can be injured. It can become diseased. The cornea, the clear part of the eye on the surface bends 10-80% of the incoming light into the eye. The lens, located inside the eye behind the cornea, is responsible for the focusing and bends 20-30% of the incoming light. The eye length, from front to back, can also affect where the light focuses. However, most vision problems are not diseases but refractive errors. These are minor flaws in the way the eye is built that causes it to focus incorrectly.
Refractive errors tend to be inherited, but there is no pattern of inheritance. Size of the eyeball, shape of the cornea, shape of the lens, and the depth of the anterior chamber are all variables in refractive errors. These variables increase the possible ocular combinations for refractive errors.
The following are some common refractive errors:
Myopia (near-sightedness)means that the distance vision of a person is blurred but the close vision is still good ( except in high degrees of myopia ). In myopia, distant objects are blurry because the images focus in front of the retina, instead of on it. Myopia affects those mostly between the ages of 10 and 40. Beyond 45 years of age myopia tends to decrease.
The unit value used to measure the degree of myopia is called Diopters which relates to the power of the spectacle lenses used to correct it. For comparison purposes the average myopic person is approximately - 3.00 diopters ( minus refers to myopia ) The higher the negative number, the higher the degree of myopia.
With high myopia (-6.00 and up) one is at increased risk of glaucoma, retinal detachments and posterior vitreous detachments.
Degenerative myopia (sometimes also called progressive myopia) is similar to simple myopia except that the degenerative changes occur in the optic disk, choroid and retina, sclera, and vitreous, and are not related to the degree of myopia. Loss of central vision, retinal detachment, and vitreous opacities are typical; cataracts and secondary glaucoma may be additional complications. Progressive myopia is genetically determined as a recessive hint.
Hyperopia (far-sightedness) means that the near vision of a person is blurred but the distant vision is still good ( except in high degrees of hyperopia ). In hyperopia, near objects are blurry because the images focus behind the retina, instead of on it.
The unit value used to measure the degree of hyperopia is called Diopters. The dioptric values for hyperopia are noted as positive. The higher the positive number, the higher the degree of hyperopia.
A young person may be farsighted, but unaware of it, because a soft lens if able to accommodate for the lack of focusing power. With age, the lens begins to harden and hyperopia becomes apparent. Young children tend to be hyperopic to some degree although the severity lessens and they age and the eye grows and becomes longer.