Anisometropia defines a situation in which the left and right eye are affected by different refractive defects. It can also be same defect in both eyes, but with completely different values. Each eye needs to achieve different degrees of accommodation, however different accommodation between the two eyes at the same time can not be achieved.

Anisometropia is divided into several groups

Hyperopic anisometropia - both eyes are affected by farsightedness, but each with different value.

Myopic anisometropia - both eyes are affected by myopia (nearsightednes­s), but each eye suffers to a different degree.

Mixed anisometropia - one eye suffers from farsightedness and the other from nearsightedness.

Astigmatic anisometropia - each eye suffers from different high degree of astigmatism.

Assuming that the difference of refraction between the two eyes is greater than 3 dioptres, anisometropia can be exclusively corrected by contact lenses.