I n d e p e n d e n t      O p t i c i a n s

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Common Vision Defects

 

 

The Emmetropic Eye

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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 eye 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 the eye's 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.

The Myopic Eye

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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. The further away an object is 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 spectacle or contact lens, a myopic eye can once again enjoy clear vision at all distances. Lenses to correct myopia tend to be thin in the centre and become thicker towards the edge.

The Hyperopic Eye

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Hyperopia or long sightedness means the closer an object is, the more blurred it appears uncorrected.

Hyperopia may 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 blurred vision.

The eye often "strains" in an effort to compensate for this defect, but in the long term this usually leads to visual fatigue, headaches and various other problems.

With an appropriate lens, a hyperopic eye can see clearly again at all distances without strain.

Lenses to correct hyperopia tend to be thicker at the centre than at the edge. The back surface is relatively flat, while the front surface has a more pronounced curvature.

The Astigmatic Eye

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Astigmatism is a condition where there is usually a variation in the shape of the eyes outer front surface, 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.

This tends to make vertical lines more clear than horizontal lines, or vice versa.

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 that they fall exactly together 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 cross-section of a ring.

The Presbyopic Eye

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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 common condition known as presbyopia.

This does not usually affect the distance vision at all, just close up vision from the end of your arms to closer.

Presbyopia is where the accommodation of the eye is no longer sufficient to see objects clearly at the usual reading distance. This is because the lenses inside the eyes gradually starts to lose their elasticity.

However, this does not usually become noticeable until about the age of 45: Having to hold a newspaper at arm's length is one of the first symptoms!


Simple reading glasses will allow the eyes to see clearly up close once again.