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Eye and Vision
Refractive Errors
Good vision has its origin in the sharp focusing of images on the retina of the eye.
The optical system of the eye, which consists of cornea, lens and vitreous body, bends rays of light (images which we see) in a way that they come to focus exactly on the retina of the eye, normally on the point of sharp imaging. This bending of light rays is called refraction. The different components of the optical system comprise the refractive power which is indicated in diopter.
| Normal Vision (Emmetropia) |
Eyes with normal vision see images at a close distance as well as images at a far distance clearly. The point of refraction is exactly on the retina.
If the point of refraction is not on the retina but in front of it or behind it, blurry images will be produced. This is called a refractive error or is also referred to as bad vision. We differentiate between:
— Nearsightedness (Myopia)
— Farsightedness (Hyperopia)
— Astigmatism (Astigmatismus)
— Presbyopia ).
Farsighted individuals are able to see objects at a far distance sharply whereas objects at a close distance appear blurry.
Compared to the normal eye, the farsighted eye usually has grown too short. The refractive point of incoming light rays is behind rather than on the retina.Eyeglasses or contact lenses for correction of farsightedness usually have collective lenses with a correction value in "plus" (convex) diopter, for example +3.5 dptr.
The farsighted eye differs from the nearsighted eye insofar as up to a certain age farsightedness can be accommodated by the lens of the eye. The eye lens is able to change shape and thereby increases its refractive power (accommodation), which makes up for the decreased refractive power of the farsighted eye for distance vision. Farsighted individuals usually are able to see images at a distance well, even at an older age. Eyeglasses for younger people are needed only in severe cases of farsightedness. On the other hand, the inability to read "presbyopia" occurs at a younger age.
| Astigmatism |
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Images of objects at a close as well as at a far distance appear distorted: A dot in real life will produce an image of a line. Usually, the reason for this is the shape of the cornea which resembles more the shape of an egg than that of a ball. The result is that light rays are bent in different directions and do not come to focus on one refractive point on the retina but at different points. |
| Presbyopia |
With advancing age, the ability of each person to change the shape of the eye lens, in order to focus correctly on images at different distances, decreases. This context is especially important if you are nearsighted. If, for example, you are slightly nearsighted (approximately –2 to –3 diopter), you will always need eyeglasses for correct distant vision. With advancing age, these glasses will also be needed. For reading purposes, the glasses can simply be removed; the individual is able to read without a corrective device. With slight nearsightedness it is possible to read without glasses at an advanced age. For distance vision (such as driving a car, watching television, etc.) eyeglasses are always needed. Should the existing nearsightedness be surgically corrected, no eyeglasses are needed for distant vision. With advanced age, however, presbyopia will appear as it would with any person, meaning that reading glasses will become necessary. At the present time, surgery for the correction of presbyopia is not considered a treatment alternative. But research has made some promising approaches to this problem |
