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Explanation of vision defects
Nearsightedness (myopia) People with myopia can see clearly up to a certain distance, beyond which objects begin to appear fuzzy or out of focus. In this case the eye has become elongated, compared to the normal, spherical eye. This causes the focal point of incoming light to not lie on the retina, but in front of it. The corrective value for the refractive error is stated in negative dioptres, for example -5.75 D. The more extreme the nearsightedness, the higher the (negative) value and the thicker the optical glasses needed. In order to focus better, the focal point must be shifted towards the retina at the back of the eye. Farsightedness (hyperopia) People with hyperopia can see distant objects clearly, but closer objects appear blurred. In the case of hyperopia the eye is usually too short; light entering the eye does not come to focus until behind the retina. In contrast to the nearsighted eye, until a certain age hyperopia can be offset by adjustment of the natural lens. The natural lens can change its shape (accommodate), increasing refractive power and compensating for the deficient refractive power of the hyperopic eye when looking at distant objects. Farsighted people are thus usually able to see clearly into the distance up to an advanced age. Only in the case of extreme farsightedness do glasses need to be worn from childhood onwards. On the other hand, the reading ability of farsighted people deteriorates at an earlier age, i.e. presbyopia becomes a problem prematurely. The solution for wearers of glasses or contact lenses is a convergent lens with a corrective value expressed in "plus" dioptres. The prescription card will contain an entry such as +3.5 D. Uneven curvature of the cornea (astigmatism) All objects, both near and far, are seen distorted. What is really a point is perceived as a line. The word astigmatism is, in fact, derived from the Greek meaning "no point". The cause is usually the shape of the cornea, which is more like an egg than a sphere. This irregular shape prevents light from focusing properly at one point on the back of the eye. Instead, it disperses on the retina. Uneven curvature of the cornea often occurs together with near- and farsightedness, and its corrective value is also measured in dioptres. This deficiency is corrected by restoring the eye's normal curvature and its ability bring images into focus. In the case of glasses this is achieved with so-called cylindrical lenses, which must be precisely aligned in front of the eye. This is why the corrective value for glasses or contact lenses is expressed as the cylinder value in dioptres and the position of the cylinder axis in angular degrees. The prescription card may in this case have an entry such as: Cyl. -1.5 D, axis 0°. Presbyopia With increasing age, everybody will experience a diminished ability of the natural lens to accommodate, making it increasingly difficult for the eye automatically focus at different ranges. This relationship is important, particularly for nearsighted people. For example, those who are nearsighted (approx. -2 to -3 dioptres) will always need glasses to focus well in the distance. These glasses are still required as the years advance. However, for reading they can simply be removed, because the wearer is able to read without a seeing aid. Later in life, slight nearsightedness thus enables one to read without glasses. Vision correction is still needed for seeing into the distance (i.e. driving a car, watching TV). If the existing nearsightedness is adjusted by means of surgery, glasses are no longer needed to focus on distant objects. However, beyond the age of approx. 45 everyone will notice the effects of presbyopia, which make reading glasses indispensable. No completely satisfactory methods of surgical correction of presbyopia are currently available. However, research on a number of approaches has already produced promising results. 
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Vision defects - LaserTravel
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