A certain number of conditions affecting the vision (myopia, hypermetropia and astigmatism) are a defect of the optic system of the eye: either it is not convergent enough or it is too convergent, or it has an irregular shape, or the optic system (lens + retina) can be normal but it is the eye that has an unusual size.
Alternatively, the myopic eye can be unusually big (the distance lens-retina is very important) with a normal lens, which gives the same result.
A myopic eye forms the image of a distant object in front of the retina. The person can not see a distant object but sees clearly a nearby object.
Alternatively, the hypermetropic eye can be too small (the distance of the lens-retina is too small) with a normal lens which gives the same result.
A hypermetropic eye can also accommodate to see an object in the distance. An hypermetropic eye cannot see well nearby but it sees well at distance.
If there is a light hypermetropia, the eye can compensate for its effect by additional accommodation: it can see distant objects precisely, it only needs to accommodate whereas a normal eye does not need to.
One of the consequences of this particularity is that the hypermetropia that is not corrected can induce headache. A light hypermetropia can go under diagnosed until the presbyopia decreases the power of accommodation of the eye.
An astigmatic eye usually has a cornea more in the shape of a “rugby ball” than of a “football ball”. This deformation is characterised by two axes, generally perpendiculars, characteristic of the deformation.
If we compare an eye to a camera or a recording camera: it is the “autofocus” function of the eye that is diminished. We say that the presbyopic eye does not accommodate enough. The objects close up seem blurred.
Presbyopia usually appears over the age of forty (even before if hypermetropia is not corrected because a part of the accommodative reserve is used for far sight). The presbyopia becomes really incapacitating around the age of 45. It increases until around 65 years old.
Presbyopia is expressed by an incapability of near vision for reading, writing, sewing… For example, a presbyopic person holds his newspaper far away to be able to read it. Finally, the arms become too short or the text too small!