AROUND THE EYE
Our most important sensory organ
The eye’s function is to receive optical stimuli and transmit them to our brain.
Good vision, on the other hand, originates from the projection of sharp images onto the retina of the eye.
The eye’s optical system, consisting of the cornea, lens, and vitreous humor, deflects incoming light rays (the images we see) so that they converge at a focal point located precisely on the retina. This is usually the point of sharpest vision. This deflection is called refraction.
The light emitted by the objects we perceive first reaches the transparent layer on the surface of the eye (cornea), is then refracted by the lens and thus focused onto the retina inside the eye.
There, the light is perceived by the nerves embedded in the retina and transmitted via the optic nerve to the perception centers in the brain, where these signals are converted into images.
The total refractive power, which is given in diopters (dpt), results from the various components of the eye’s optical system.
The normal eye (normal vision = emmetropia)
In a person with normal vision, light rays are focused onto the pigment layer of the retina to form a sharp image when looking into the distance. This image, scanned by our retina with its almost infinite number of point-like sensory cells, is laterally inverted and upside down, similar to the image on photographic film. Our brain processes the visual information from both eyes into a unified visual impression, a representation of what we see – and want to see.
The structure of the eye is very similar to that of a camera (see below). The cornea acts like a glass lens and corresponds to the front lens of the camera lens. The pupil corresponds to the aperture of the camera. The lens of the eye corresponds to the other glass lenses built into a camera lens. Finally, the retina is comparable to the film in a camera.
Refractive Errors
If the focal point is not exactly on the retina, i.e., in front of or behind it, the surroundings are seen as blurry. This is called a refractive error or refractive error, colloquially also referred to as a vision problem.
In Central Europe, approximately 15-20% of the population is nearsighted and approximately 5% is farsighted.
The nearsighted eye (nearsightedness = myopia)
You see clearly at close range, but objects further away appear blurry.
Your eye is too long, unlike the almost spherical shape of a normal eye. The focal point of the incoming light rays in your eye lies not on, but in front of the retina.
The correction value for the refractive error of your eye is given in this case with negative diopter values, e.g. -5.75 dpt.
The stronger the nearsightedness, the higher the (negative) value and the thicker the lenses.
To improve your vision and sharpen it, the focal point must be shifted backward onto the retina. Optical correction using laser correction is achieved by reducing the refractive power of the cornea.
The farsighted eye (farsightedness = hyperopia)
You can see distant objects clearly, but you can only see up close indistinctly.
In farsightedness, the eye is usually too short, so the light rays are focused behind the retina.
Unlike nearsightedness, farsightedness can be compensated for by the eye’s lens up to a certain age. The lens can change shape (accommodation) and thereby increase its refractive power, compensating for the insufficient refractive power of the farsighted eye when looking at distant objects.
People with farsightedness usually maintain good distance vision well into old age. Only in cases of extreme farsightedness do they need to wear glasses in their youth. On the other hand, farsighted people experience a decline in reading ability earlier; that is, age-related farsightedness (presbyopia) becomes noticeable sooner.
Laser correction is achieved by increasing the refractive power of the optical system. In farsightedness, the eye is usually too short, so the light rays are focused behind the retina.
Astigmatism (corneal curvature = astigmatism)
They see objects near and far as distorted.
A point in nature appears as a line in your perception. Hence the expression “asymmetrical vision”.
The reason for this is usually the cornea of your eye, which is more egg-shaped than sphere. As a result, the light rays are refracted differently and do not converge on a single point, but are scattered before reaching the retina.
Astigmatism often occurs together with nearsightedness or farsightedness; its correction value is also given in diopters.
It is corrected by reshaping the corneal surface to a more spherical shape, so that the light rays converge at a single point. Laser correction is achieved by correcting the curvature, ensuring that all light rays meet at a single focal point.
Age-related farsightedness (presbyopia)
As people age, the ability of the eye lens to automatically focus on different distances diminishes.
These connections are particularly important for short-sighted people.
For example, someone with mild nearsightedness (around -2 to -3 diopters) always needs glasses to see clearly at a distance. These glasses are also necessary in old age. However, they can simply be removed for reading, allowing the person to read without any aids.
Mild nearsightedness allows reading without glasses in old age. However, correction is always necessary for distance vision (i.e., driving, watching television, etc.).
If existing nearsightedness is corrected surgically, clear distance vision is possible without glasses. However, with increasing age, starting around age 45, presbyopia develops – as in everyone else – and reading glasses become necessary.
Presbyopia can be corrected surgically using multifocal or trifocal lens implantation. You can find more information here.
What type of visual impairment do I have?
Your eyeglass prescription will tell you whether you have a combination of nearsightedness or farsightedness and astigmatism, and how severe the refractive error is.
Explanations
R / L: Values for the right / left eye
Sph (sphere): Indicates your diopter value. Nearsightedness is given in negative values, e.g., -5.75 diopters (nearsighted), farsightedness in positive values, e.g., +3.5 diopters (farsighted).
Cyl (cylinder): Indicates the value of your astigmatism in diopters, e.g. 1.5 dpt. Positive or negative sign is irrelevant here.
Axis: Indicates the axis of your astigmatism in degrees. In our example, the astigmatism in one eye has a direction of 120 degrees.
Add: Shows the value for presbyopia once it sets in. The value +2 diopters is added to the sphere correction for near vision.
