Visual refractive errors
The visual process begins with the formation of an optical image of the outside world on the photoreceptive cells of the retina. Since the formation of an optical image is the first true step in visual perception, every imperfection in the optical apparatus has the potential to seriously deteriorate eyesight.
The optical system of the human eye is made up of three main components: the cornea, the pupil, and the crystalline lens
For simplicity's sake, the eye's lenses - the cornea and the crystalline lens – can be likened to camera lenses, and the retina to photographic film.
The cornea is the first and most important ocular optics, determining about two-thirds of the total optical power of the eye. The crystalline lens is placed immediately behind the pupil; anchored, through the zonular fibers, to the ciliary body. The tension generated on the zonular fibers by the ciliary muscle induces an increase in curvature of the surfaces of the lens allowing the focusing of objects. This capacity is called accommodation. The pupil determines the amount of light entering the eye; it is now known that the amount of visual blurring is directly correlated to the pupil diameter. Wider pupils induce greater blurring of the retinal images (see Figure below). Images of the outside world, through diopters, are focused on the retina.
The pupil dilatation increases the blurring of the images projected on the retina (as highlighted in the simulation of an eight-typical table: at the top for a pupil of 2 mm, at the bottom for a pupil of 7 mm), mostly due to the increase in the amount of optical aberrations depending on the pupil diameter itself.
Optical aberrations are the main cause of visual cloudiness. They are classically distinguished in low order aberrations, correctable with ordinary glasses or contact lenses and in high order aberrations, not correctable with traditional optics. Among the low order aberrations there are the "classic" optical defects of the human eye, which we will descibein this section, such as myopia, hypermetropia and astigmatism. Presbyopia is a defect linked to the aging of the accommodation process, which leads to a progressive reduction in the focus of near images after 45 years of age.
In emmetropic (normal) eyes, light rays come into focus directly on the retina, and vision is sharp.
In myopic eyes, light rays come into focus in front of the retina instead of on the retina itself. This can be caused a longer-than-normal axial length of the eye (axial myopia), excessive curvature of the cornea, or an increase in the index of refraction of the crystalline lens. In most cases, myopia is axial in nature. Myopia is corrected with concave spherical lenses.
In hyperopia, under relaxed accommodation, light rays focus behind the retina. In order to focus the light rays on the retina, it is necessary to augment the dioptric power of the eye through an "accommodative effort" or by using a convex spherical lens. Continuous accommodative effort can lead to asthenopia, such as eye pain, the rapid onset of fatigue during close-up work, conjunctival hyperemia, and blepharitis. Hyperopia can be due to a shorter-than-normal axial length of the eye, lower-than-normal corneal curvature, or variation in the index of refraction in the crystalline lens (such as in cataracts, for example).
Astigmatism is a condition in which the asymmetry of ocular refraction produces an unequal refraction of light rays in the various meridians; for this reason a ray of light, instead of focusing on a single point, is focused on two different focus planes at a 90 degree angle to each other, one anterior and one posterior. Astigmatism can be myopic, hyperopic, or mixed.
Presbyopia consists of the physiological reduction of the accommodation of the crystalline lens due to advancing age, the end result being the blurring of near objects. There are two competing theories, one which holds that the gradual loss of accommodation is due to the inadequate contraction of ciliary muscles as age advances, and the other pointing to a gradual increase of the crystalline lens in older subjects. The end result is the same, however, and most people over the age of 45 must wear eyeglasses for reading.