The visual defects or amétropies , are disorders of the refraction due to an anomaly of the optical system formed by the Cornée, the Cristallin and the Rétine. In a normal eye, the focal point of the cornea-crystalline lens unit is located on the retina. In vision of close, the capacity of Accommodation of the crystalline lens (which bulges), makes it possible to advance the focal point in front of the retina to preserve a concentration on the retina of the luminous rays which are not parallel any more but divergent. The absence of visual defect names emmétropie .

• Myopia: the focal point is located in front of the retina. The remote objects appear fuzzy.

• Hypermétropie : the focal point is located behind the retina. The hypermétrope is obliged to adapt permanently to advance the focal point, the close objects appear fuzzy when the capacities of accommodation are exceeded.
• Astigmatism: the luminous rays are focused in two distinct hearths on the retina. This is generally caused by a irregularity of the cornea which involves a defect of Stigmatisme.
• Presbyopia: reduction in the capacity of accommodation of the crystalline lens of to ageing. The close objects appear fuzzy because the crystalline lens is not able any more to adapt sufficiently.

They can be corrected by the wearing of corrective lenses, contact lenses or by an surgical operation.

See too

Internal bonds

• Punctum proximum

• Punctum remotum
• Myopia
• Hypermétropie
• Presbyopia
• Lens (optical)
• Ophthalmology
• Optician

External bonds

• defects of vision, explanations, vision and correction