Common Vision Defects

THE EMMETROPIC EYE-ALL DISTANCES

The picture shows what is known as an "emmetropic" eye, i.e. it has sharply focused vision at all distances.

In emmetropic eyes the refractive power of the cornea and lens is exactly matched to the length of the eye. Light rays are deflected in the eye in such a way that they fall exactly on the retina, producing a sharp image.

With the aid of its crystalline lens, the eye can adapt to all distances in much the same way as a zoom camera lens. This process is known as accommodation.

MYOPIA (SHORTSIGHTEDNESS)

Myopia or shortsightedness is a condition where the eye is too long in relation to its refractive power. The light rays therefore intersect in front of the retina instead of on it. Blurred vision results. The further away the object observed, the more blurred it appears. Clear vision is limited to nearby objects. Shortsighted therefore means having good near vision but poor distance vision.

With an appropriate eyeglass lens, a myopic eye can once again enjoy clear vision at all distances. As you see here, the light rays are slightly diverged in front of the eye, shifting their point of intersection to the surface of the retina. This creates the same conditions as those enjoyed by emmetropic eyes, providing the wearer with excellent vision.

A lens for myopia is a diverging or minus lens. Minus lenses are always thinner at the centre than at the edge. The front surface is relatively flat, while the back surface has a more pronounced curvature. The higher the power required, the thicker the edge becomes and the heavier the lens.

HYPEROPIC (LONGSIGHTEDNESS)

Hyperopia or long sightedness means the closer an object is, the more blurred it appears uncorrected, hyperopia bring on fatigue, sore eyes, headaches and eyestrain. The eye is too short in relation to its refractive power. The light rays intersect behind the retina, resulting in a blurred vision. An attempt is sometimes made to compensate for this defect by accommodation, but in the long term this leads to problems.

With an appropriate lens, a hyperopic eye can see clearly again at all distances without strain. As you see, the light rays are converged in front of the eye, shifting their point of intersection to the surface of the retina. Accommodation can once again be used solely for adjusting to different distances.

A lens for hyperopia is a converging or plus lens. Plus lenses are always thicker at the centre than at the edge. The back surface is relatively flat, while the front surface has a more pronounced curvature. The higher the power required, the thicker the centre becomes and the heavier the lens.

THE ASTIGMATIC EYE-DISTORTED VISION

Astigmatism is a condition where there is a variation in the shape of the cornea. Rays of light pass through the cornea on two planes - vertical and horizontal. In the astigmatic eye one corneal plane is steeper than the other, so that the rays of light do not come together at the same point. One point can be in front of the retina, the other behind. Both can be in front of the retina, etc.

The more unequal the curvature of the cornea, the more blurred your vision will be.
With an appropriate lens, an astigmatic eye can once again enjoy clear, undistorted vision. The rays of light are deflected in such a way they fall exactly on the retina.
A lens for astigmatism is a cylindrical lens, i.e. it is not spherical in shape but has a different curvature in two directions. It can be more easily imagined as the section of a ring.

THE PRESBYOPIC EYE-READING DISTANCE

If you are over 40 and finding it difficult to read because your eyes can no longer focus at short distances, you may have a condition known as presbyopia.

Presbyopia is a condition where the accommodation of the eye is no longer sufficient to see objects clearly at the usual reading distance of approx. 40 cm. This is due to the fact that the lens of the eye gradually starts to lose its elasticity from the day we are born. However, this does not become noticeable until about the age of 45: having to hold a newspaper at arm's length is one of the first symptoms!