This section addresses refraction problems that require optical correction as well as the main problems that limit visual quality, even though they are not necessarily associated with the need for prescription glasses.
AMETROPIAS
Myopia
Hyperopia
Astigmatism
Presbyopia

 

 

 

 

 

AMETROPIAS
In the first place, a distinction should be made between emmetropia and ametropia.

An emmetropic eye is one in which the image of an object at an infinite distance is formed on the retina and accommodated without any effort. In theory, an emmetropic eye has 60 dioptres and an axial length of 22 mm. The axial length is the distance between the cornea and the retina. In this case, we say that the eye's remote point-of-gaze is in the infinite.

An ametropic eye is one in which the image of an object at an infinite distance is not formed on the retina. Vision is not sharp and needs to be corrected. There are three kinds of ametropia: myopia, hyperopia and astigmatism.

There are two possible causes for ametropia:
• Refractive ametropia: An emmetropic eye has 60 dioptres, which makes it possible to see sharp images of objects at an infinite distance without any special effort. An ametropic eye may have more or less than 60 dioptres and therefore requires a correcting lens to provide sharp vision at any distance.

• Axial ametropia: The inability to focus properly may be due to a deviation in the axial length, so this difference also needs to be corrected with a lens. Ametropia may also consist of a combination of refractive and axial ametropia.

 

Myopia
Definition: Myopia or nearsightedness is a refractive defect of the eye in which rays of light converge in front of the retina, causing distant objects to become blurred. This may be because the eye’s axial length is longer than usual and/or its power is greater.
Symptoms: The main signs and symptoms of myopia or nearsightedness are as follows: blurred vision of distant objects, good near vision, large or protruding eyes, mydriasis (dilatation of the pupil), deviation of the visual axis of one eye away from the other eye (exophoria or exotropia), short working distance in very near vision, inhibition of peripheral visual field, limited eye motility, etc.
Correction: Myopia can be corrected with a negative or diverging lens. Such lenses can be fitted into frames or adapted as contact lenses. Contrary to positive lenses, diverging lenses increase their effective power when placed near the eye. This is why many myopes using hypocorrected lenses have the habit of placing their glasses nearer to their eyes when looking at an object in the distance.

 

Hyperopia
Definition: Definition: Hyperopia or farsightedness is a refractive defect of the eye in which rays of light converge at a point behind the retina. This may be because the eye’s axial length is shorter than usual or because the power of the visual system is lower. It is common for children to be born with slight hyperopia but, due to the development of the human eye during growth, children's eyes can frequently become normal by the time they are six.
Symptoms: The main signs and symptoms of hyperopia are as follows: headaches, problems focusing on near objects, learning problems, photophobia, feeling of being cross-eyed, myosis (pupillary constriction), small or sunken eyes, etc. When the hyperopia is not very severe, the eyes may take advantage of their accommodative mechanism to compensate for the refractive error and thereby see things well without blurring.
Correction: Hyperopia can be corrected with a converging or positive lens. These lenses are also used to correct accommodative problems and aphakia (cases of people who have undergone cataract operations).

 

Astigmatismo
Definition:Astigmatism is a defect of the eye in which one meridian is of maximum power and another of minimum power. The degree of astigmatism usually remains stable throughout a person’s life. If it varies at all, it does so very slightly, particularly when it is due to the shape of the cornea. Astigmatism is the most frequent form of ametropia. It is very often hereditary and can affect myopes and hyperopes alike.
Symptoms: The most common signs and symptoms of astigmatism are as follows: asthenopia (headache, watery eyes, photophobia, itching eyes, etc.), dizziness, vertigo, intermittent blurry vision in the near-vision field, giddiness when changing fields of vision from far to near, etc.
Correction: Astigmatism is corrected with toric or astigmatism lenses.

 

Presbyopia
Definition: Presbyopia is a disorder that happens to everyone and involves needing glasses to read. It is caused by a decline in the amplitude of accommodation. The crystalline lens, which is responsible for focusing, loses elasticity and cannot fulfil its function properly. People usually begin to suffer from presbyopia at the age of 40-45.
Symptoms: The main symptom of presbyopia is the inability to perform visual tasks in the near-vision field. Presbyopes usually have to hold objects at a distance from the eyes (a problem that is accentuated when there is little light), and may also be associated with asthenopia and headaches.
Correction: Presbyopia is progressive and usually becomes stable around the age of 65. To be able to see near objects clearly, presbyopes need lenses with a specific prescription. There are several kinds of lenses available for people who begin to need glasses due to presbyopia: conventional near-vision glasses, reading glasses, bifocals, trifocals and progressives.