|The vision is the function thanks to which an object forms an image on the retina. This object which is carried out by the eye, is transmitted through the optical path. The image forms itself thanks to the entrance of luminous shades which go through the cornea to reach the retina.|
The image of a distant object forms either in front of or behind the retina. There are different types of ametropias. Ametropia is an abnormal refractive condition (myopia, hyperopia or astigmatism) of the eye in which images fail to focus upon the retina.
Distant objects are fairly clear but images close up are blurred: correction is carried out using positive strength convex lenses.
Hypermetropia is a visual fault due to an insufficiently strong eye (“too short”). The image is formed behind the retina, which explains why a hypermetropic person sees better at long- distance than at short-distance.
Distant objects are blurred while those which are closer are clear; correction is carried out using negative strength concave lenses.
Myopia is a visual fault due to the eye being too long: the distance between the cornea and the retina is too large. The image is formed in front of the retina and the myopic person sees poorly at long-distance but well at short-distance.
This ametropia is the most common, and due to an irregularity of the curve of the cornea; vision of objects is thus deformed and imprecise, both at short and long-distance. The eye needs to be corrected in a different way, in two perpendicular directions; the correction is carried out through toric lenses, which can be negative or positive.
Astigmatism is a visual fault generally due to the curve of the cornea, which is slightly oval instead of being round.
Presbyopia is not a real ametropia. It is a natural development of sight, which affects everyone from the age of 40 onwards. For the 20 million people in France who already have presbyopia, short-sightedness results from a loss of suppleness in the crystalline which curves insufficiently and is therefore unable to adjust easily. The result is a gradually increasing difficulty in near-distance vision.
The ability to adapt has become insufficient, making it difficult to focus on close objects (when reading for example)