Complete Guide
A big difference between your two eyes — why do new glasses make you dizzier?
Anisometropia and aniseikonia
Your two eyes are several diopters apart. Each eye tests clear on its own, yet with both eyes in the glasses you feel dizzy and text jumps as you read. When that happens, the power isn't necessarily wrong.
- ✦A big difference between the two eyes' prescriptions is anisometropia; a difference in the size of the images the two eyes see is aniseikonia.
- ✦Image size is affected by power, base curve, center thickness and vertex distance.
- ✦When you look down to read, the prism in the two lenses differs too.
- ✦If a child has a big difference between the two eyes, see an ophthalmologist first.
Aniseikonia: your two eyes see the same world at different sizes
Normally, when both eyes look at the same object, the two images sent to the brain are close to the same size, and the brain fuses them into one with little effort while extracting depth from the tiny differences. Aniseikonia means the two eyes see images that are noticeably different in size — the brain is forced to fuse one larger and one smaller picture, which is tiring work.
Picture two projectors set to different magnifications, projecting the same film onto the same wall: the images overlap but never quite line up, and watching it for long tires both your eyes and your head. People with aniseikonia often can't put their finger on what is wrong — they only feel that "these glasses just aren't right."
The most common cause is a big difference between the two prescriptions (anisometropia). When one eye is -1.00D and the other is -4.00D, even if both eyes are each corrected to clear vision, the magnification produced through the glasses differs, and a size difference appears. Cataract surgery in one eye can also suddenly open up a big difference between the two eyes; when glasses can be fitted after surgery should follow the ophthalmologist's advice.
Common symptoms with a big difference between the two eyes
The situations below are not necessarily all aniseikonia — they can also come from prescription, cylinder axis, PD, or binocular coordination issues. If several apply at once, it's worth having both eyes measured together.
Dizziness and aching temples after long reading
After long stretches of reading or screen work, a dull pressure between the brows or at the temples, and the urge to close your eyes and rest
Text jumps while reading; skipping lines
While reading books or documents the characters seem to sway, and you find yourself jumping to the next line
Misjudging distance on stairs or when reversing
Nearly missing a step going downstairs, misjudging distance when reversing the car, reaching for something and being slightly off
New glasses feel worse, not better
The new prescription is more accurate, yet the new glasses leave you dizzier and more tired
Wanting to close one eye after a while
Squinting or closing one eye without realizing it, because using both eyes together is too tiring
If the symptoms come on suddenly, if you start seeing double, or if a child shows these signs, please see an ophthalmologist first.
The 4 things spectacle magnification depends on
The magnification of a pair of glasses is mainly affected by four factors: lens power, lens base curve, lens center thickness, and vertex distance. When the two eyes' prescriptions differ a lot, our optometry staff first measure how much the two eyes' image sizes differ, then decide whether — with the prescription kept correct — to specify the lenses' base curve and center thickness to bring the two eyes' magnification closer, into a range that is easier to fuse. How much closer depends on the size of the difference and the frame, and only measuring tells.
① Lens power
Power is the lead actor in magnification. The larger the difference in power, the more noticeable the gap in magnification between the two eyes usually is.
② Lens base curve
The curvature of the front surface of the lens. At the same power, a steeper or flatter front surface changes magnification — this is the shape factor.
③ Lens center thickness
The thicker the center of the lens, the more the shape factor magnifies. Center thickness is also linked to refractive index and frame size, so the three are worked out together.
④ Vertex distance
The distance from lens to eye. Using the formula Fc = F ÷ (1 − dF), a -6.00D lens shifts by about 0.04D in effective power for every 1mm, and the higher the power the bigger the shift. If the two sides of the frame sit unevenly, the difference between the eyes widens.
Where the difference comes from matters too. By Knapp's law, in theory, when the difference comes mainly from the length of the eyeball (axial), spectacles leave a smaller image-size difference; when it comes mainly from the cornea or crystalline lens (refractive), contact lenses bring the images closer. Real eyes don't always follow the theory, so it still has to be measured.
What we look at together when fitting glasses for a big difference between the two eyes
Once each eye's prescription is measured, the two eyes are looked at together as a pair.
Binocular vision assessment
Both stores include a binocular vision assessment in every exam, and ask how your day splits between distance, near and screen work. If corrected vision stays low however the prescription is changed, optometry staff refer you to an ophthalmologist under Article 12, Paragraph 3 of Taiwan's Optometrists Act
Vertex distance and frame symmetry
A big difference between the eyes is especially sensitive to vertex distance; the distance of each lens from the eye is measured, and the nose pads and temples are adjusted to be symmetrical
Lens design parameter planning
First we measure how much the two eyes' image sizes differ; where needed, with the prescription kept correct, we specify base curve, center thickness and refractive index to bring the two eyes' magnification closer. How much closer depends on the size of the difference and the frame
Contact lenses as an option
Contact lenses move with the eye, so looking down to read brings no prism difference between two lenses; whether image sizes end up closer than with spectacles depends on where the difference comes from (see Knapp's law above). Suitability is assessed in person; under Article 12 of Taiwan's Optometrists Act, contact lens fitting for those aged fifteen and under must be carried out under the guidance of an ophthalmologist, and no fitting is provided for children under six
Children see an ophthalmologist first
A big difference between the two eyes is linked to amblyopia and strabismus, whose diagnosis and treatment belong to ophthalmology. Under Article 12 of the Optometrists Act, children under six may not be examined by optometry staff, and exams for those fifteen and under are carried out under the guidance of an ophthalmologist
Follow-up within 2 weeks of pickup
Both eyes need time to get used to new glasses. Within 2 weeks of pickup, the store proactively follows up on how they feel; for glasses made with us, frame adjustments after pickup carry no extra charge
Image sizes can be brought closer, but not made identical
The base curve, the center thickness and the distance between lens and eye described above can each bring the two eyes' image sizes a little closer, but only up to a limit. Base curve and center thickness cannot be adjusted without end: push them too far and the lens gets thicker and heavier, and clarity has to be looked after as well. Frame size, face shape and the height of your nose bridge also limit how close the lens can sit to the eye. The bigger the difference between the two eyes, the bigger the gap that usually remains in the end.
So what this pair of glasses aims for is to bring the difference between the two eyes' images into a range the brain finds easier to fuse, not to make them identical. When the store follows up, just tell us honestly how reading, going downstairs and screen work each feel.
A big difference between the two eyes: frequently asked questions
Q: My two eyes have very different prescriptions — why do I get dizzy and see text jump after a while?
With a big difference between the two prescriptions, the left and right images you see through your glasses can differ in size even when each eye on its own is corrected clearly; and when you look down to read, the prism effect in the lower half of the two lenses is larger for one eye than the other. The brain has to merge two images that differ a little in size and position, and over time this easily shows up as dizziness, tired eyes, and text that jumps or line-skipping. It is an optical effect of spectacle magnification and prism, but prescription, cylinder axis and PD can cause similar feelings too, so it takes measuring to tell them apart.
Q: Why does vertex distance matter so much for people with a big difference between the two eyes?
Vertex distance is the distance from the back surface of the lens to the cornea; when it changes, both effective power and image magnification change. Using the formula Fc = F ÷ (1 − dF), a -6.00D lens shifts by about 0.04D in effective power for every 1mm and a -10.00D lens by about 0.10D — the higher the power, the bigger the shift. For someone with a big difference between the two eyes, if the two lenses sit at different distances from the eyes, or the frame sits crooked, the gap between the two eyes widens further. That is why vertex distance is measured and both sides of the frame are adjusted to be symmetrical.
Q: Are contact lenses less likely to cause aniseikonia?
It depends on where the difference between the two eyes comes from. By Knapp's law, in theory, if the difference comes mainly from the cornea or the crystalline lens, image sizes are closer with contact lenses; if it comes mainly from the length of the eyeball, spectacles may actually bring them closer. Contact lenses also differ when you look down to read: the lens moves with the eye, so there is no difference in prism between the lower halves of two spectacle lenses. Whether contacts suit you depends on corneal condition, tear film and daily routine, and is assessed in person by optometry staff. Under Article 12 of Taiwan's Optometrists Act, contact lens fitting for those aged fifteen and under must be carried out under the guidance of an ophthalmologist, and no fitting is provided for children under six.
Q: Does aniseikonia go away?
Separate two situations. If the image-size difference comes from the spectacle prescription and lens parameters, it is an optical matter: what fitting can do is bring the two eyes' images closer so they are easier to fuse — that is not a cure. If an eye disease is suspected behind it, such as a cataract or a retinal problem, diagnosis and treatment belong to ophthalmology, so please see an ophthalmologist first.
Q: Where in Xinzhuang or Banqiao can I get glasses assessed for a big difference between the two eyes?
At either of our two stores: the Xinzhuang store, Beyond Visual Optometry (BiYang Optometry Clinic), No. 419-1, Zhongzheng Rd., Xinzhuang Dist., New Taipei City, 02-2206-6700; and the Banqiao store, Beyond Visual Optometry × Neng Jian Du Optometry Clinic, No. 28, Ln. 47, Sec. 2, Shuangshi Rd., Banqiao Dist., 02-2253-1246. Both stores include a binocular vision assessment in every exam, measure vertex distance, monocular PD and fitting height, and measure how much the two eyes' image sizes differ when needed. If a child has a big difference between the two eyes, please see an ophthalmologist first.
About the author and sources
YoYo (Cheng Chia-Yu), Optometrist
Optometrist License No. 003309 | Graduate of Yuanpei University of Medical Technology | Lecturer, New Taipei City Optometrists Association
Lead optometrist at Beyond Visual Optometry × BiYang Optometry Clinic, specializing in clinical vision exams, binocular vision assessment, progressive lens fitting and children's myopia tracking.
Meet Optometrist YoYo →
Sources
- Rabbetts RB. Bennett & Rabbetts' Clinical Visual Optics. 4th ed. Butterworth-Heinemann; 2007 (spectacle magnification = shape factor × power factor, Knapp's law, aniseikonia)
- Jalie M. The Principles of Ophthalmic Lenses. 5th ed. Association of British Dispensing Opticians (vertex-distance conversion Fc = F ÷ (1 − dF), Prentice's rule)
- American Academy of Ophthalmology EyeWiki: Aniseikonia (definition and causes)
- Beyond Visual: What Is Vertex Distance? (conversion table)
- Optometrists Act (Laws & Regulations Database of the ROC, in Chinese; Article 12 on exams for those fifteen and under, contact lens fitting and referral)
The magnification, vertex-distance and prism figures were worked out with the optical formulas above; the axial versus refractive distinction follows Knapp's law and describes the theoretical case. Whether our stores can specify base curve and center thickness, whether they measure the two eyes' image-size difference, and that both stores include a binocular vision assessment in every exam were confirmed by our stores before being written here; how much closer the images can be brought is decided by in-store measurement. Diagnosis and treatment of amblyopia, strabismus and other eye diseases should be assessed by an ophthalmologist. Last updated: October 24, 2026.
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The hard part is wearing them with both eyes, so both eyes are measured together
The power of each eye, the size of the image each eye sees and where the frame sits are all done on the same day; please allow 60–90 minutes, at Xinzhuang or Banqiao. If the glasses are for a child, have an ophthalmologist see them first.
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