For an optically accurate image of objects, their sharp representation on the retina is necessary. For this, light rays entering the eye have to be focused on the retina. The cornea and lens are in charge of this collecting of light rays in the eye. If the focus of the collected light rays is not on the retina, it is called defective vision or ametropia.
There are different kinds of defective vision: Near-sightedness (myopia), far-sightedness (hyperopia), astigmatism and presbyopia. Defective vision can be corrected with glasses, contact lenses, and, in suitable cases, by surgery.
In near-sightedness, the focus is in front of the retina. The refractive power of the cornea and lens is too strong, and/or the eyeball is too long. Divergent lenses are needed to correct this refraction anomaly. Thus, a prescription for glasses could be, e.g., for -5.0 spherical (diopters).
In far-sightedness, the focus is behind the retina. The refractive power of the cornea and lens is too weak, and/or the eyeball is too short. Convergent lenses are needed to correct this refraction anomaly. Thus, a prescription for glasses could be, e.g., for + 4.0 spherical (diopters).
In astigmatism, the refractive power of cornea and/or lens are not equal in all planes. They are not formed like an exact sphere, and therefore result in different refraction in different planes of the eye. In regular astigmatism, the plane with the strongest refractive power and the plane with the weakest refractive power form a right angle. For this reason, a dot is represented on the retina as a rod.
This refractive error can be corrected with cylindrical glasses. The cylinder is oriented towards the dominant plane of the eye. Of course, near-sightedness or far-sightedness may be present as well. Thus, a prescription for glasses could be, e.g., for -2.0 spherical and +1.5 cylindrical, +60°.
Our lens continuously loses its ability to change shape, and thus, the ability to sharply represent the objects that are close by. This change of shape in the lens, which enables us to focus on objects at different distances, is called accomodation. From approximately the 45th year of age, it will become necessary to wear reading glasses for seeing up close. Far-sighted patients usually need reading glasses somewhat earlier, near-sighted patients frequently can do without reading glasses.
More refractive power is needed to focus up close than in the distance. Thus, a prescription for reading glasses could be, e.g., for +3.0 spherical.