Vision problems and their correction

Myopia and hyperopia

An eye is said to be emmetropic when it naturally forms a sharp image on the retina, without accommodation, for an object at infinity. Some eyes have defects known as ametropia.

• Myopia: the image is formed in front of the retina (the eye is too long or the lens is too convergent). The focal point is at a finite distance rather than at infinity. Correction: diverging lens.
• Hypermetropia: the image is formed behind the retina (the eye is too short). The eye must accommodate even to see at infinity. Correction: converging lens.

Presbyopia and astigmatism

With age, the lens loses its flexibility: this is presbyopia, which increases the near point as accommodation decreases. It affects everyone over the age of 40–45, even those with emmetropia. Astigmatism results from a non-spherical cornea: focus varies depending on the direction (vertical or horizontal), and is corrected using cylindrical lenses.

Summary table

Defect	Cause	Consequence	Correction
Myopia	Eye converges too much	Blurred distance vision	Diverging lens
Hyperopia	Eye does not converge enough	Blurred near vision, eye strain	Converging lens
Presbyopia	Loss of accommodation	Near point recedes	Converging lens (magnifying glass)

Common pitfall

Do not confuse hyperopia with presbyopia: hypermetropia is a defect linked to the shape of the eye, sometimes present from birth, whereas presbyopia is a natural ageing of the lens that affects everyone, even those with myopia, who may then see clearly from a distance without glasses but need glasses for near vision.