The lens system of the eye

1. Normal vision (emmetropia)

A sharp image on the retina of the eye is a prerequisite for good vision. The optical system of the eye can be likened to that of a camera and consists of the cornea, the lens and the vitreous body. The pupil functions as a shutter. The cornea and lens refract incoming light rays originating from the objects surrounding us in such a way that the light rays originating from a point on the object come together again at a point on the retina. If they do not meet at a point on the retina but spread over a larger area, the image is unclear.

In ideal circumstances, sharp distant and close-up images are reproduced on the retina without assistance. If a point of an object is reproduced at a point in front of the retina, this is described as nearsightedness. If the point lies behind the retina the eye is farsighted. If the focal length is different in different directions, it is impossible to focus the image at a single point on the retina, resulting in a line image. This is known as astigmatism.

2. Nearsightedness (myopia)

In the case of nearsightedness the eye is generally too long. As a consequence, the incoming light rays are focussed in front of the retina, in other words the focal point is in front of the retina. The longer the eye, the greater the nearsightedness and the more indistinct the image.

Correction of the defective vision by means of glasses or contact lenses is carried out by a diverging lens (concave lens), the corrective value of which is expressed in minus-diopters. This moves the focal point backwards onto the retina.

3. Farsightedness (Hyperopia)

Farsightedness is also known as long-sightedness. Because the eyeball is generally shorter in this case, the light rays are only focussed behind the retina, so that the focal point is behind the retina. A farsighted eye is usually able to see clearly at a distance, provided it can adapt the focal length of its lens through accommodation. However, close-up images become blurred when the accommodation range is exhausted. Through increasing the refractive power, the focal point of the eye must be moved forward onto the retina. Correction of the defective vision by means of glasses or contact lenses is carried out by a converging lens (convex lens), the corrective value of which is expressed in plus-diopters.

4. Astigmatism

The curvature of the cornea and lens is not even, but changes slightly in different directions. As a consequence the focal length also varies in different directions. In order to bring all light rays together at a single point, the uneven curvature must be compensated for. The compensation is achieved by means of so-called cylindrical lenses that must be positioned in a precisely determined direction in front of the eye.

Hence with glasses and contact lenses the correction value is stated as the value of the cylinder in diopters together with the position of the cylinder axis in degrees e.g. cylinder – 2.5 diopters, axis 80º.

5. Presbyopia

Over the course of the years the natural lenses loose their elasticity and thus the ability to focus the eye sharply at different distances (accommodation). Presbyopia is a natural aging process of the lens and generally becomes apparent around 45 - 50 years of age. It happens to everyone without exception. The normally sighted person then requires reading glasses, while the long-sighted person requires distance and reading glasses. In contrast to a normally sighted person the slightly shortsighted individual simply removes his or her glasses to read. This fact should be taken into account if you are perhaps considering undergoing laser treatment for shortsightedness. With strongly shortsighted eyes a small correction is necessary for close up vision in order to be able to see clearly at a comfortable distance. At the moment, there is still no sensible operative correction available for presbyopia.