Definition and Purpose

2.2 Nature of Refractive Corneal Surgery

The basic concepts about the cornea and how to change its shape are introduced.

2.2.1 Vision Concept

About 70 % of the refraction in the human eye is done by the cornea, other parts by the fluids inside the eye and a reserved portion of refraction by the eye's adjustable lens to guarantee vision at both short and long distance. The human vision system seems to be designed rather for long distance than for short distance, as its working point without any additional power lies more in far vision. With age the feature of vision at short distances is lost slowly with a breakpoint between 40 and 48 years; people then need convex lenses to be able to read newspapers etc.. For instance children, can see sharply between 10 cm of up to infinity, adults in their thirties from about 20 cm to infinity, adults in their fifties from 40 cm to infinity and retired people over 70 only see sharply in infinity.

Farsightedness or nearsightedness means that the normal working point of the human vision system is not (close) at long distance for some reason. Maybe, for genetic reasons and/or alientening the eye for constant near vision in the case of nearsightedness. However, depending on how much the working point is out of position, the adjustable lens inside the eye can only cover a limited range within the boundaries of normal vision.

In emmetropia -working point in infinity- the refraction system of the eye combines the light rays from infinity exactly in the retina. The radius of the refraction system represents the degree of refraction achieved, depending on the length of the eye the optimal radius varies in each eye in order to produce emmetropia. So if the rays of infinity are not bounded that they actually meet the retina in a point, either the length of the eye must be changed -practically rather impossible- or the refraction must be changed so that the rays meet properly at the retina. This means the overall radius of the lens system must be changed. Therefore we will always talk about an optimal radius in relation to the size of the eye.