Information on visual defects
THE HEALTHY EYE
How does a healthy eye work?
When looking at an object in the distance a normal-sighted eye will focus the light rays entering the eye onto a pigment layer of the retina to produce a sharply focussed image. The image is sensed by a virtually uncountable number of point-form sensory cells and, just as in a camera, it is upside down and the wrong way round. Our brain processes the visual information from both eyes to produce a unified visual image, the image of what we see, and what we want to see.
Is it possible to focus sharply on both near and distant objects?
Up to the age of roughly 40 we can adjust the lens in our eyes to produce sharp images of both near and distant objects. This ability is called accommodation.
When exactly happens during accommodation?
In accommodation, similar to the focal length setting on a camera, the eye probably shifts slightly the position of the lens within the eye whilst changing its curvature, making it thicker in the middle, thereby refracting the light more strongly (close-up vision). In this way even close up objects can be viewed sharply in focus.
Accommodation works automatically, why don't we notice it?
The accommodation process starts out with an unfocused image which our brain is not satisfied with. This takes place as quickly as the nerve cells permit the transmission of signals. All of this happens subconsciously.
LONG-SIGHTEDNESS - HYPEROPIA
What happens in a long-sighted eye?
A long-sighted eye is too short. The light rays would not converge until behind the retina. The image appears blurred. Up to the ages of roughly 35 to 40 you can often still see clearly because the lens can adapt. This adaptability of the lens is called accommodation.
Correction of long-sightedness
What possibilities are there today?
Spectacles (convex lenses),
and contact lenses,
Long-sightedness can also be corrected by means of an operation. Once corrected you will be able to see clear, focussed images again.
What about for children, should they wear glasses?
Above a certain level (1.5 to 2 dioptres and upwards) children should wear spectacles, especially young children. This is because otherwise they can suffer from convergent squint if there is not correction from an early stage.
SHORT-SIGHTEDNESS - MYOPIA
What happens in a short-sighted eye?
The light rays would converge in front of the retina to produce a sharp image. By the time the rays hit the retina they are no longer in focus and so the sufferer sees an image which is to a greater or lesser extent blurred.
Correction of myopia
What possibilities are there?
Short-sightedness can be corrected by:
spectacles with concave lenses,
contact lenses,
by an eye operation. After correction the patient will again be able to see clear, sharply focussed images.
What about children, should they wear glasses?
This depends on the age and on other factors. A decision can only be made here in consultation with the eye specialist. Above a certain level (1.5 to 2 dioptres) children should wear spectacles.
What is the explanation for the increase in short-sightedness?
Unfortunately we still know too little about the causes. In most cases the cause seems to be genetic, and just as with body height, this can only be influenced to a limited extent by external factors. In recent years there has been increase in those who believe that there is a link between intensive work at PC monitors and the increase in short-sightedness in adults.
Is there really nothing you can do to influence myopia?
One of the few things that you can do is change your contact lenses to highly gas-permeable hard lenses, where this is possible.
ASTIGMATISM
What happens in an astigmatic eye?
An astigmatic eye is neither too short, nor too long. With astigmatism the curvature of the cornea is uneven. The image is created both in front of and behind the retina. Because the image is deformed when it reaches the retina it is perceived as blurred to a greater or lesser degree.
What does an astigmatic image look like?
Astigmatism is a refractory disorder meaning that a point is not seen as a point, but as a rod and, depending on the direction of the astigmatism, the image is both unfocussed and distorted, e.g. stretched either in height or width.
Correction of astigmatism
What possibilities are there available today?
Astigmatism can be corrected by:
glasses with special lenses (convex/concave) "cylindrical lenses",
contact lenses,
by an eye operation. After correction the patient will again be able to see clear, sharply focussed images.
What about children, should they wear glasses?
Above a certain level (from 1.5 to 2 dioptres) they should certainly wear glasses.
Why is it so important for children?
Because a distorted image has a negative effect on our perception
Because a distorted image impairs our spatial vision
Because a distorted image quite frequently leads to weak eyesight. Such weak-sightedness can only be treated effectively in the specialist eye schools.