If the rays of light fall through the lens in parallel and if the focus is precisely on the retina of the eye, a sharp image is created on the retina. In this case, there is no defective vision. However, if the eyeball is too short or too long, the focus is in front or behind the retina and there is deficient vision.
EMMETROPIA (PERFECT VISION)
Emmetropic people can depict objects that are far away in focused form on their retina.
The rays of light fall in parallel through the lens and the focus is precisely on the retina of the eye: this creates a sharp picture on the retina.
Without correction, myopic people see objects that are far away unfocused. Close-up, they not only see well, they even see better than people without defective vision under the same conditions.
With short-sightedness, the eyeball is often too long; that is why the rays of light already meet in front of the retina. When images seen from a distance reach the retina, they appear unfocused.
Short-sightedness can have a variety of causes. The most frequent is axial myopia which is caused by the eyeball being too long. Axial myopia is normally inherited. Refractive myopia is less common and is caused by excessive refraction of the cornea or the lens. This is the case when the cornea or the lens has increased curvature or if the core of the lens is opaque (a cataract, for instance, can be responsible for the latter).
Without correction, hyperopic people see objects far away focused. However, they see close-up objects blurred. This is attributable to a refraction error that causes the focus of light rays that fall in parallel to be behind the retina of the eye. There are two types of far-sightedness.
Axial hyperopia: Most far-sighted people have an eye that is too short. All babies are far-sightedness due to their eyeball being too small. In most cases, this condition stabilises as the child grows.
Refractive hyperopia: The eye length is normal but the refraction of the eye lens is insufficient. The rays of light only come together behind the retina.
To compensate for the refraction error that causes far-sightedness, a non-corrected far-sighted eye adapts (i.e. it accommodates). The muscle work means that it changes the shape of the lens. The higher the dioptres, the more the musculoskeletal system of the eye has to make an effort. Headaches, etc. are the result.
With increasing age, the ability of the lens to accommodate declines. This is called presbyopia.
AGE-RELATED DEFECTIVE VISION (PRESBYOPIA)
The lens of the eye bundles the light that falls through the pupil into the eye and regulates it like an auto zoom. It makes it possible for an eye with correct vision to focus from very close-up to infinity. With increasing age, the core of the lens loses its elasticity and thus the ability to adapt close-up by accommodating. Sharp vision close-up is thus no longer possible without suitable correction. However, age-related defective vision is not an illness but rather a normal loss of function due to age.