Complete Guide
What should you watch for when getting glasses for high myopia?
Measurements, lens thickness, astigmatism, contact lenses and eye exams
Above -6.00D, the things that need extra care when getting glasses are only a handful, but each of them leaves less room for error. Optometrist YoYo wrote this for people with deep myopia who are about to get a new pair.
- ✦Above -6.00D, small errors in vertex distance, monocular PD and fitting height are magnified by the power.
- ✦Lens thickness depends on the frame and your PD first, and the index second.
- ✦High astigmatism and a big difference between the two eyes each need their own attention.
- ✦A fundus examination belongs to ophthalmology — please have regular check-ups with an ophthalmologist.
Above -6.00D, why is fitting glasses different from usual?
The core reason is the amplification of optical error at high prescriptions. The same parameter error is barely felt at -2.00D but can become obvious visual distortion, dizziness, or discomfort at -8.00D. The parameter with the largest impact is vertex distance: the distance from the back surface of the lens to the apex of the cornea.
During refraction, the phoropter's vertex distance is about 12–13mm; but the frame you actually wear lands anywhere from 8mm to 16mm depending on nose-pad height and face shape. The closer the lens sits to the eye, the stronger the actual correction; the farther, the weaker — and the higher the prescription, the more pronounced this difference:
| Rx Power | Vertex Distance 12mm | Effective at 8mm | Difference |
|---|---|---|---|
| -6.00D | -6.00D | approx. -5.86D | approx. 0.14D |
| -10.00D | -10.00D | approx. -9.62D | approx. 0.38D |
| -14.00D | -14.00D | approx. -13.26D | approx. 0.74D |
Formula: Fc = F ÷ (1 − dF), where F is the measured power and d is how far the lens moves toward the eye, in meters. For -6.00D: -6.00 ÷ [1 − 0.004 × (-6.00)] = -6.00 ÷ 1.024 ≈ -5.86D, a difference of about 0.14D. Moving the same 4mm at -14.00D changes it by about 0.74D.
The same prescription gives different actual correction at different wearing positions. This is why "the prescription is clearly right, but distance vision is a bit foggy once fitted" is especially common in high myopia — a high-myopia prescription must have vertex-distance compensation calculated for the actual frame wearing position.
Another amplification effect is the prism effect: the visual shift produced when the line of sight strays from the lens's optical center, with a magnitude equal to the power times the decentration (in centimeters). The same 3mm optical-center offset produces only 0.6Δ at -2.00D — imperceptible to most — but 2.4Δ at -8.00D, enough to make the two eyes work harder to fuse the image. So high-myopia fitting must precisely measure monocular PD and fitting height to align the optical center with your visual axis.
How to choose a lens index: 1.60, 1.67, 1.74
Many people assume high myopia means going straight to 1.74, but the frame comes first: each step up in index trims roughly a tenth off the extra edge thickness.
| Index | Where it usually fits | What to keep in mind |
|---|---|---|
| 1.60 | Up to about -4.00D, 1.60 is usually enough | Going one step higher at this power saves very little thickness |
| 1.67 | From -4.00D to -6.00D, look at the frame first | Its Abbe number is the lowest of the three, so people sensitive to color fringing should say so |
| 1.74 | Above -6.00D, provided the frame is already small | The highest index among the brands we carry |
For a -9.00D office worker in glasses more than 12 hours a day, the weight 1.74 shaves off is felt on the bridge of the nose all day; but for someone at -5.00D, the same budget put into a smaller frame plus 1.67 often yields a thinner, lighter result. Index should be judged by prescription, frame size, face shape, and visual needs together.
Frame choice: often more impactful on thickness than the index
A myopic lens is thin in the middle and thick at the edge; how thick that edge ends up depends first on frame size and your PD.
Frame size and decentration
The wider the frame, the farther from the optical center the lens has to be cut, leaving exactly the thickest ring behind. As an example, the same power in a 48mm frame and a 54mm frame ends up with clearly different edge thickness, and a left-right symmetrical shape keeps the two lenses balanced.
Frame center distance vs. your PD
The frame center distance is the distance between the centers of the two lens openings. The further it is from your PD, the further the optical center has to be shifted, and the thicker the outer edge of a myopic lens becomes. Factor the PD in when choosing the frame.
Thick acetate, stable fit
A thicker acetate rim can conceal part of the lens's edge thickness. The frame must also sit snug without sliding — once it slips, the vertex distance changes, and high myopia feels it directly. Adjustable nose pads help if your nose bridge is low.
Still uncomfortable in your glasses? Sometimes the problem isn't the prescription but the frame and lens design: Still Uncomfortable in Your Glasses? The 3 Real Causes and Fixes
Glasses, soft contacts, RGP: three options for high myopia
None is always better — it depends on your eyes and how you spend your day.
| Option | For high myopia | Keep in mind |
|---|---|---|
| Glasses | Easy to put on and take off; astigmatism correction stays stable | Thicker edges and more weight at high powers; the field is limited by the frame, and things look slightly smaller than they are |
| Soft contact lenses | No frame in the way; vertex distance near zero, so the power is usually weaker than the glasses prescription | With high astigmatism a soft lens may rotate and the axis drifts; wearing time and dry eye are assessed in person |
| RGP rigid contact lenses | Custom-made to the shape of the cornea; another option for high astigmatism | Needs an adaptation period; both stores fit RGP (Xinzhuang uses a TOPCON CA-800 corneal topographer, Banqiao has a slit lamp) |
An often-overlooked detail: from about -4.00D upward, contact-lens power needs to be converted via vertex distance and is usually weaker than the spectacle prescription — buying contacts straight from a glasses prescription may over-correct, so we recommend confirming it through professional fitting. 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.
A deeper comparison: Contact Lenses vs. Glasses: Which Is Better? · RGP in detail: The Complete Guide to RGP Rigid Contact Lenses
The extra measurements for high myopia
A high prescription magnifies every millimeter of error, so a few measurements get extra care. Both stores include a binocular vision assessment in every exam.
Vertex-distance measurement and compensation
Converts effective power for the actual frame wearing position you chose, rather than copying the phoropter's numbers
Monocular PD (MPD-L / MPD-R)
Each eye measured separately, not total PD divided by two — at high prescriptions, a 1–2mm offset is enough to produce a perceptible prism effect
Fitting height
Measured with the chosen frame on; if the frame changes, it's measured again
Subjective confirmation of cylinder axis (JCC)
The autorefractor's axis is only a starting value. A 10° axis error leaves about one third of the cylinder uncorrected, so the axis is confirmed subjectively when high myopia comes with astigmatism
Your old glasses
Bring them along: a lensmeter reads the old lenses' power and optical centers, so the old and new can be compared
Follow-up within 2 weeks of pickup
Within 2 weeks of pickup, the store proactively follows up on how the glasses feel. For glasses made with us, frame adjustments after pickup carry no extra charge
-6.00D is a number. It is also an eyeball that has been stretched.
Vertex-distance conversion, monocular PD, choosing a smaller frame: everything earlier on this page is about where light lands after it passes through the lens. It deals with the power, which is a number. But a power this strong usually means the eyeball has grown longer from front to back, and the retina has been stretched thinner along with it. A lens moves the light; the eye is still the same eye.
According to public health-education materials from Taiwan's Health Promotion Administration and the American Academy of Ophthalmology's EyeWiki, people with high myopia carry a higher later risk of retinal detachment, macular degeneration and glaucoma than other people. These conditions often bring no obvious symptoms at first, so please have regular check-ups with an ophthalmologist.
Some changes should not wait. If floaters or flashes suddenly increase, see a doctor the same day; if part of your field of vision suddenly goes missing, or your vision suddenly blurs, please see an ophthalmologist as soon as possible. For where to go first with other symptoms, Eye Doctor or Optometrist? A Symptom Triage Guide has a 30-second triage. If, during the exam, corrected vision will not come up however the lenses are changed, licensed optometry staff will refer you to an ophthalmologist under Article 12, Paragraph 3 of Taiwan's Optometrists Act.
High-myopia glasses: frequently asked questions
Q: Can I get thin lenses at -10.00D?
Yes. A myopic lens is thin in the middle, with its thickness piled up around the edge. To thin that edge at -10.00D, do three things together: choose a smaller frame, pick a frame whose center distance is close to your monocular PD, and go up to a 1.74 index. Among the brands we carry, resin lenses go up to an index of 1.74.
Q: How strong does myopia have to be to count as high myopia?
High myopia is generally defined as -6.00D or stronger; the International Myopia Institute (IMI) used -6.00D as the cut-off in its 2019 definitions. At this level, small errors in vertex distance, monocular PD and fitting height are all magnified by the power. As for eye health, according to public health-education materials from Taiwan's Health Promotion Administration and the American Academy of Ophthalmology's EyeWiki, high myopia carries a higher later risk of retinal disease, so please have regular check-ups with an ophthalmologist.
Q: How often should someone with high myopia see an ophthalmologist?
For high myopia, how often to be examined, and whether the fundus should be examined as well, is decided by your ophthalmologist based on your eyes. If floaters or flashes suddenly increase, don't wait for the next appointment — see a doctor the same day. A vision exam measures your prescription and binocular vision; it cannot replace an ophthalmologist's examination.
Q: Where in Xinzhuang or Banqiao can I get high-myopia glasses?
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, near Xinzhuang MRT station; 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, near Jiangzicui MRT station. Both stores measure vertex distance, monocular PD and fitting height, include a binocular vision assessment in every exam, and fit RGP rigid contact lenses. If glasses made elsewhere feel uncomfortable, bring them in and our optometry staff will look at them with you.
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
- Flitcroft DI, et al. IMI – Defining and Classifying Myopia. Investigative Ophthalmology & Visual Science. 2019 (high myopia defined at -6.00D)
- Health Promotion Administration, Ministry of Health and Welfare: "Myopia is a disease" public-education poster, 2018 (high myopia and cataract, glaucoma, macular degeneration and retinal detachment; in Chinese)
- American Academy of Ophthalmology EyeWiki: Pathologic Myopia (Myopic Degeneration); Retinal Detachment; Primary Open-Angle Glaucoma
- Jalie M. The Principles of Ophthalmic Lenses. 5th ed. Association of British Dispensing Opticians (vertex-distance conversion Fc = F ÷ (1 − dF), Prentice's rule, lens thickness and index)
- Beyond Visual: What Is Vertex Distance? (conversion table)
- Optometrists Act (Laws & Regulations Database of the ROC, in Chinese)
The vertex-distance, prism and thickness figures were calculated with the optical formulas above; the definition of high myopia comes from the International Myopia Institute; the eye-disease risks follow public health-education materials from Taiwan's Health Promotion Administration and the American Academy of Ophthalmology's EyeWiki, and how often the eyes are examined is for the ophthalmologist to decide. What both stores measure, pickup times, and which store has the CA-800 or the slit lamp were confirmed by our stores before being written here. Diagnosis and treatment of eye disease should be assessed by an ophthalmologist. Last updated: October 25, 2026.
Articles on high myopia
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A millimeter or two can be enough to feel, so give the measuring time
Choose the frame and the lenses on the same day, so the measurements can follow that frame; please allow 60–90 minutes when you book. Both Xinzhuang and Banqiao take bookings, and don't leave your old pair at home.
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