Optical medical prescriptions often contain complicated technical terms, which are not always easy to understand for a novice. However, we need this information in order to produce your glasses.
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The basic anatomy of the eye
This is the first lens of the eye. The cornea helps process our vision.
The crystalline lens of the eye
This is the interior lens of the eye which allows us a neat, clear vision from close-by as well as from far. This focus is established thanks to the flexibility of the crystalline lens, like the autofocus of a camera. This phenomenon is called "accommodation".
The retina is the inner coat of the eye, and is made out of several layers of nerve cells (rods and cones), which transform the image shaped by the eye via a nerve impulse to the brain.
The defects of our sight
Myopia is a visual defect which essentially affects the vision of objects that are far away. It means that the image of objects is formed in front of the retina instead of on the retina. Therefore the person has a blurred vision of objects that are far away, but has a good near vision.
The higher the level of myopia, the shorter the distance from which the myopic has a clear vision.
Myopia or nearsightedness is corrected with a negative lens (divergent or concave). The negative lens is thick on the edges and thin in the centre. The principle is to push the image of the object onto the retina in order to re-establish a clear vision.
Hyperopia is a defect of vision caused by an imperfection in the eye, affecting the vision of nearby objects. The image of objects is formed at the back of the retina instead of on the retina, which causes a blurred vision of objects that are closeby.
In weak cases of hyperopia, the person has a good near-vision, but the continuous effort can cause tired eyes.
Hyperopia is corrected with a positive lens (convergent or convex). Positive lenses are thicker in the centre and thinner on the edges. Therefore, the image of the objects is placed again correctly on the retina.
Astigmatism is a relatively common visual defect. With astigmatism, the corneal curvature is slightly more oval, while the curve should be round. It causes a blurred vision from close-by as well as from far.
Astigmatism can be associated to myopia, hyperopia or presbyopia.
Astigmatism can be corrected by cylindrical lenses or toric lenses. The thickness of the lens varies according the strength needed.
Presbyopia is not a defect of the vision. It is rather part of a natural aging process. Presbyopia occurs from the age of forty and is inevitable. This phenomenon is due to the aging of the crystalline lens, which causes a decline in the accommodation abilities of the eye (causing difficulties of focus). The nearby vision becomes gradually more challenging.
Presbyopia can be corrected with different kinds of lenses :
- Simple reading glasses will correct your near-vision, a distance of circa 40 centimetres. To see far, you constantly need to lower or take off your glasses.
- Proximity glasses, also called office lenses, allow you to have a clear vision from close-by, while keeping a clear vision of the people around you. They are generally used for computer and office work.
- Progressive lenses, here we speak about progressive lenses.
The elements on the prescription
The sphere is a dioptric value.
The sphere describes the degree of myopia when preceded by the symbol -, and the degree of hyperopia when preceded by a+.
The Cylinder is a dioptric value. It compensates astigmatism.
The cylinder value is not required in all cases. It is only necessary when there is a deformation of the cornea. If an eyeglass prescription includes cylinder power, it also must include an axis value. The cylinder value is generally the second value on the left side of your prescription and can have a negative or positive symbol.
The Axis (Ax) is important when you choose a lens.
We need the value of the axis to indicate the position of the cylinder on the lenses of your glasses.
The axis value indicates in what direction the lenses should be positioned in the frame, where the lens should be thicker and where it should be thinner. The axis is defined with a number from 1 to 180. It is not preceded by a negative or positive symbol, and is indicated in degrees, for example: 178°. This value has been communicated to you by your ophthalmologist or your optician. The axis value is generally the third value on the left side of your prescription. There is always two values: one for the right eye and one for left.
You only have to fill out an axis value if you also have a cylinder value.
The ADD (or addition) is the difference between far vision Sphere and near vision Sphere. It is mandatory we have this value in order for us to create progressive or degressive glasses.
Prisms are prescribed to compensate for eye alignment problems that can bring about diplopia (double vision).