The quality of ophthalmic optics

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The perfect cooperation of the light-refracting units of the eye is necessary for the formation of a sharp image on the retina. These are the cornea and the lens. Their task is to focus the light rays arriving from different angles and to refract them in such a way that they do not simply continue straight ahead but meet in the fovea centralis. Diopters (dpt, the reciprocal of the focal length dpt=1/f) express the refractive power as approximately 43 dpt for the cornea and 19 dpt for the lens. This results in a total refractive power of 65 diopters for the normally sighted eye. Diseases that under- or exceed this refractive power lead to a blurred retinal image, which in turn reduces the resolving power of the visual system. Below, we briefly outline the most common eye diseases that cause this.

If the lens has too little refractive power or the eyeball is too short, the sharp image in the eye does not appear until it is behind the retina. This is called farsightedness or hyperopia. The juvenile eye can compensate for this for a very long time by increased near vision (accommodation). However, glasses or contact lenses correct farsightedness to prevent eye pain and headaches caused by this over-exertion. Conversely, nearsightedness or myopia occurs when the eyeball is too long or the lens’s refractive power is too high, forming a sharp image in front of the retina. A visual aid also corrects this defective vision.

In order for us to see distant and close objects sharply, the lens must adapt its shape to the respective distance. This process is called accommodation. Many people gradually lose the necessary elasticity in their lenses between the ages of 40 and 50, a normal aging process known as presbyopia. If we frequently read and force the lens to adjust to short distances, it can lead to a decrease in visual acuity during the day. This is because the lack of elasticity only allows the accommodation to fully release overnight. In this case, the visual acuity for distant objects will be better in the morning, after waking up, than in the evening. Of course, the reverse can also occur, in which the visual acuity for near vision deteriorates throughout the day due to presbyopia. In both cases, reading glasses or glasses for distance vision can be of great help.

Astigmatism is an eye disease caused by an irregular curvature of the cornea. This can be congenital or caused by scars after corneal injuries. In any case, the retina cannot bundle the light rays entering the eye into a single point. For this reason, a point is not perceived as a point but as a blurred line (rod). The remedy is glasses with cylindrical lenses or dimensionally stable contact lenses.

Clouding of the lens of the eye (cataract) is 90% a symptom of old age but can also occur after eye injuries, exposure to radiation, as a side effect of medication, in diabetes mellitus or congenitally after a prenatal infection (e.g., rubella). Symptoms include slowly increasing visual disturbances and severe glare. In the advanced stages, affected individuals claim to see as if through a frosted glass. The most common therapy is surgery under local anesthesia.

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