Anisometropia and Aniseikonia: Prescribing High and Unequal Powers
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
When the two eyes have different prescriptions, glasses can make the images different in size and create unequal prism as the eyes move. Children risk amblyopia, adults risk headache and double vision. Knowing when to choose contact lenses, a partial correction or a special lens design solves most of these problems.
Definitions
- Anisometropia: a difference in refraction between the eyes, usually considered significant at 1.00 D or more (spherical equivalent or cylinder).
- Aniseikonia: a difference in the perceived size or shape of the two retinal images. About 1% per dioptre of anisometropia in spectacles; most people tolerate up to about 3–5% (so up to about 2–3 D difference in glasses).
- Antimetropia: one eye myopic and the other hyperopic.
Why it matters
| Problem | Cause | Management |
|---|---|---|
| Anisometropic amblyopia | The more hyperopic (or more astigmatic) eye is never in focus in early childhood | Full-time correction early; patching or atropine if needed (see amblyopia) |
| Aniseikonia | Spectacle magnification differs between eyes (plus magnifies, minus minifies) | Contact lenses, refractive surgery, or iseikonic lenses (adjusting base curve, thickness and vertex) |
| Vertical prism imbalance at near | Reading through points below the optical centres of unequal lenses (Prentice) | Slab-off (bicentric grinding), dissimilar segments, lowered optical centres, or separate reading glasses |
| Reduced binocular vision and stereopsis | Unequal images and amblyopia | Correct early; contact lenses in adults |
Worked example: vertical imbalance
R −1.00 DS, L −4.00 DS. Reading 8 mm below the optical centres: R 0.8 × 1 = 0.8Δ base down, L 0.8 × 4 = 3.2Δ base down. The difference is 2.4Δ, more than the usual tolerance of about 1–1.5Δ, so the patient may get near diplopia or strain. A slab-off on the more minus lens (the left) can remove about 2–2.5Δ.
Spectacle magnification in short
- Spectacle magnification depends on the power factor (power and vertex distance) and the shape factor (front curve and thickness).
- Moving a plus lens closer to the eye reduces magnification; moving a minus lens closer reduces minification. Contact lenses (vertex zero) give the smallest image size difference for refractive anisometropia.
- Knapp's law predicts that spectacles suit pure axial anisometropia, but in practice contact lenses are usually better tolerated because retinal stretching in long eyes offsets image size.
Prescribing high powers
| Prescription | Practical points |
|---|---|
| High myopia | High-index lenses with small eye-size frames and minimal decentration; lenticular or "myodisc" designs over about −12 to −15 D; contact lenses give larger images and better field; record vertex distance |
| High hyperopia and aphakia | Aspheric lenticular designs; small round frames; ring scotoma and jack-in-the-box effect with aphakic spectacles; contact lenses or IOL preferred |
| Anisometropia after cataract surgery in one eye | Temporary contact lens, or plan the second eye soon |
| High astigmatism | Small frames, careful axis and fitting; toric contact lenses |
Always measure and state the vertex distance for powers over ±4.00 D, centre lenses on the pupils, and use the patient's monocular PDs and heights.
Practical pearls
- In a child, a 1.50 D hyperopic anisometropia with reduced vision in that eye is amblyopia until proved otherwise: prescribe the full correction and review vision.
- For an adult with near complaints from unequal lenses, compare prism at the reading position before blaming the add.
- Most adult anisometropes over about 3 D are happier in contact lenses for distance, with glasses over them for near if needed.
Where students slip
- Forgetting vertical prism imbalance when giving bifocals or progressives to anisometropes.
- Prescribing a big anisometropic correction to an adult who has never worn it without a trial.
- Thinking aniseikonia is only about power: vertex, base curve and thickness also matter.
Test yourself: flip cards
Quick quiz
Related: Refraction Calculators: Near Add, AC/A Ratio, Retinoscopy and Prism, Prentice's Rule: Induced Prism With Worked Examples, Vertex Distance: Formula, Examples and When It Matters, Amblyopia: Classification, Severity and PEDIG Treatment Lessons, Contact Lens Calculator: Soft, Toric and RGP Power, Base Curve and Tear Lens. Back to the optometry curriculum.
Key sources
- Rabbetts RB. Bennett and Rabbetts' Clinical Visual Optics. 4th ed. Butterworth-Heinemann; 2007
- Jalie M. Ophthalmic Lenses and Dispensing. 3rd ed. Butterworth-Heinemann; 2008
- Pediatric Eye Disease Investigator Group. Treatment of anisometropic amblyopia in children with refractive correction. Ophthalmology. 2006;113:895-903
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