Anisometropia is the condition in which the two eye

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

s have unequal refractive power; that is, are in different states of myopia

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

 (nearsightedness), hyperopia

Hyperopia

Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

 (farsightedness) or in the extreme, antimetropia (wherein one eye is myopic and the other is hyperopic), the unequal refractive states cause unequal rotations thus leading to diplopia

Diplopia

Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...

 and asthenopia

Asthenopia

Asthenopia or eye strain is an ophthalmological condition that manifests itself through nonspecific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache and occasional double vision...

.

Anisometropia can adversely affect the development of binocular vision

Binocular vision

Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...

 in infants and children if there is a large difference in clarity between the two eyes. The brain will often suppress the vision of the blurrier eye in a condition called amblyopia

Amblyopia

Amblyopia, also known as lazy eye, is a disorder of the visual system that is characterized by a vision deficiency in an eye that is otherwise physically normal, or out of proportion to associated structural abnormalities of the eye...

, or lazy eye.

The name is from four Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

components: an- "not," iso- "same," metr- "measure," ops "eye."

One study estimated that 6% of those between the ages of 6 and 18 have anisometropia.

Spectacle correction

For those with large degrees of anisometropia, spectacle correction may cause the person to experience a difference in image magnification between the two eyes which could also prevent the development of good binocular vision.

The solution for spectacle wearers to the problem that spectacle correction may cause the person to experience a difference in image magnification between the two eyes is spectacles incorporating iseikonic lenses. Iseikonic lenses present adjusted image sizes to the eye compared to standard lenses.
The formula for iseikonic lenses (without cylinder) is:

Magnification = 1/(1-(t/n)P) X 1/(1-hF)

where:
t = center thickness (in meters)
n = refractive index
P = front base curve
h = vertex distance (in meters)
F = back vertex power (essentially, the prescription for the lens)

(see Practical Optical Dispensing by David Wilson)

If the difference between the eyes is up to 3 diopter spheres iseikonic lenses can compensate. At a difference of 3 diopter spheres the lenses would however be very visibly different - one lens would need to be at least 3mm thicker and have a base curve increased by 7.5 spheres.

Contact lens correction

The usual recommendation for those needing iseikonic correction is to wear contact lenses in their normal prescription since the effect of vertex distance is removed and the effect of center thickness is also almost removed, meaning there is minimal and likely unnoticeable image size difference when wearing contacts. This is a good solution for those who can tolerate contact lenses and for whom the contact lenses create sufficient acuity.