What is the nature of dioptric defects?
The eye is an optical system very similar to a camera. The transparent cornea, which makes up the frontal area of the eye, is the main refractive component of this optic system. Another part of the optic system is the lens inside the eye, which represents a supplementary refractive force in the eye’s optical apparatus. In young people, the lens changes its shape enabling fine focussing when gazing at various distances.
Therefore, so that parallel light rays entering the eye hit the place of most focused vision on the retina after going through the optic system, the correct ratio of the eye’s length and the thickness of the optical system must be kept. A number of inheritable and developmental factors can lead to a structural imparity. If the eye is too long or short, light waves do not converge on the retina. This imbalance can be manifested in the form of three dioptric defects.
 Myopia (short-sightedness)
Is the most common dioptric deficiency, affecting about 30 % of the population. In myopia the eye is too long or the optic system’s refraction is too large. Light rays therefore converge in front of the area where vision is its sharpest and an unfocussed, fuzzy image falls upon the retina.
This deficiency is most often corrected by diffuse (-) spectacle lenses or contact lenses, which correct the light rays such that they meet on the retina.
 Hypermetropia (Long-sightedness)
Affects about 10 % of the population. In hypermetropia, the eye is too short or the curvature is insufficient. Light rays therefore converge behind the place of sharpest vision and blurred, foggy images fall on the retina.
This deficiency is usually corrected by convex (+) spectacle lenses or contact lenses, which correct the light rays' path enabling them to meet on the retina.
Astigmatism
 This deficiency is the inability to see sharp images at any distance due to the irregular shape of the eye's corneas. The cornea doesn't have a regular semi-circular shape but in some axes it is flat or, contrarily, more curved. Thus part of the light rays converge in a different place than on the place of sharpest vision and a blurred, foggy and deformed image falls upon the retina. Astigmatism can occur on its own or in connection with long-sightedness or short-sightedness.
This deficiency is usually corrected with cylindrical spectacles glass (the cylindrical glass diffracts the light rays in just one level) and, with difficulty, contact lenses.
Higher order aberrations
These are deviations and irregularities in the eye's optical system, which can have a great effect on the quality of sight. Among the main higher order aberrations are coma, spherical aberrations, secondary astigmatism, trefoil and quadrofoil. These aberrations are not corrected by spectacles, contact lenses or traditional laser surgery. They can be a source of significant impairments to the quality of vision (poor vision at night, dusk, double vision, ghosting etc.)
Higher order aberrations, on average, make up about 20 % of the refractive deficiencies in the eye. For a number of patients however, they make up about 50 % of the resulting reduction in vision quality. Higher order aberrations can be effected using the diagnostic apparatus LADARWave and the therapeutic methods CustomCornea wavefront LASIK, PRK and LASEK.
Presbyopia (short arm syndrome)
It is a dioptric deficiency connected with age affecting all people older than 45. It is manifested in difficulties in reading nearby objects. Up to the ages of 40 – 45, the lens in the eye has enough flexibility for the focussing muscles to affect its shape and thus the optical robustness according to the distance of the observed object. With age, the lens flexibility drops and the person loses the ability to focus on nearby objects.
This deficiency is usually solved by prescribing spectacles or the monovision technique, in which one eye is corrected for long sight and the other for short sight using contact lenses. More information on solving presbyopia can be found in the chapters PRELEX and Monovision. Recently very advanced clinical trials of new kinds of operations that are aimed at functional surgical corrections of presbyopia have been carried out. If you are interested in surgically correcting of your presbyobia, please get information on the present state of these operations from the doctors at our clinic.
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