Vision Problems: Refractive Errors

Good vision occurs when the eye properly focuses sharp images onto the retina.
The optical system of the eye, which consists of the cornea, lens and vitreous body, bends incoming light rays (images which we see) in a way that they come to a sharp focus at one point located exactly on the retina of the eye. In general, this is the point of sharpest focus.

In eyes with normal vision the focal point is located exactly on the retina. The normal eye sees images clearly at both a close distance and a far distance. If the focal point is not located exactly on the retina but rather in front of or behind it, blurry images are produced. This is referred to as a refractive error or vision disorder, or more commonly bad vision.

Normal vision / Emmetropia

In eyes with normal vision (emmetropia), the optical system of the eye, consisting of cornea, lens and vitreous body, bends rays of light (which form the images that we see) in such a way that they come to a sharp focus on the retina. The focal point is located directly on the retina.

Nearsightedness / Myopia

If you are nearsighted, you are able to see objects at a close distance clearly whereas objects at a far distance appear blurry. Compared to the eye with normal vision, which resembles the shape of a ball, your eye has grown too long. The refraction point of incoming light rays is not located on the retina itself but in front of it. In this case, the correction value for the refractive error of your eye is indicated in negative diopter numbers, for example -5.75 diopter. The more severe your nearsightedness is, the higher the (negative) value and the thicker your eyeglasses will be. The optical correction is made by reducing the refractive power of your eye. With the correction, the refractive point is moved further back onto the retina so that images will be seen sharply. For eyeglasses and contact lenses, the correction is made with a concave lens, and the corrective value is indicated in "minus" diopter. Your prescription will read for example -5.75 diopter. If surgery is performed, the refractive power of the cornea or of the lens is altered in such a way that it reaches optimal conditions. Zum Seitenanfang

Farsightedness / Hyperopia

If you are farsighted, you are able to see objects at a far distance clearly whereas objects at a close distance appear blurry. Compared to the eye with normal vision, your eye has grown too short. The refractive point of incoming light rays is not located on the retina itself but behind it. The optical correction is made by increasing the refractive power of your eye, in other words by moving the refractive point forward. For eyeglasses and contact lenses, the correction is made with a convex lens, and the corrective value is indicated in "plus" diopter. Your prescription will read for example +3.5 diopter. If surgery is performed, the refractive power of the cornea or of the lens is altered in such a way that it reaches optimal conditions. The farsighted eye differs from the nearsighted eye in the following way. The eye accommodates farsightedness until you are a certain age. The lens of the eye is able to change shape (accommodation) and thereby increases its refractive power, which compensates for the decreased refractive power of the farsighted eye for distance vision. Farsighted individuals usually are able to see images at a distance well even at advanced ages. Eyeglasses for younger people are needed only in severe cases of farsightedness. On the other hand, the inability to read, also called presbyopia, occurs at a younger age. Top of page

Astigmatism

Patients with astigmatism see objects as distorted at any distance. For people with astigmatism, the image of an actual dot is perceived as that of a line. Astigmatism often occurs together with nearsightedness or farsightedness. Usually, the reason for this condition is the shape of the cornea, which resembles the shape of an egg rather than the shape of a ball. Because of the different curvatures of the cornea, rays of light are bent differently (for example, in horizontal and vertical directions). As a result, images appear distorted. The optical correction is made by balancing the curvature of the corneal surface in such a way that when corrected it resumes the ball-like shape and light rays will come to focus at one single point of refraction. For eyeglasses and contact lenses, the correction is made with so-called cylindrical glasses. The position of the corrective lens in front of the eye needs to have a specifically defined direction. This is why the correction value for eyeglasses and contact lenses is measured in diopter for the cylinder value and in degrees for the position of the cylinder axis. Your prescription would read for example cylinder value: cyl. -1.5 dptr, axis 0°. If surgery is performed, the refractive power of the cornea is altered in such a way that it reaches optimal conditions. To top of page

Presbyopia

With advancing age, the lens of the human eye loses its ability to change its shape automatically in order to focus correctly on images at different distances. Beginning at an age of approximately 35 to 45, most people need reading glasses in order to see objects clearly at a close distance. Click here for more information on presbyopia and presbyopia correction.