Quick answer: A glasses prescription is a set of optical measurements, not a diagnosis. SPH describes the main focusing power, CYL and AXIS correct astigmatism, ADD adds near power for presbyopia, and PD helps position the lenses in front of your pupils. The signs and numbers must be interpreted together, and a glasses prescription cannot be used as a contact-lens prescription.
Key takeaway: Read the prescription to understand your lenses, but let an ophthalmologist decide whether the numbers reflect simple refractive error or a change that needs a full eye-health examination.
If you have recently had an eye test in Korea, you may see terms such as OD, OS, SPH, CYL, AXIS, ADD, PD or VA. The abbreviations can look unfamiliar, especially when you are comparing a Korean printout with a prescription from another country. This guide explains the common fields in plain English for patients in Guro, Gasan Digital Complex, Guro Digital Complex, Yeongdeungpo, Gwanak and nearby parts of Seoul.
First: which eye is OD and which eye is OS?
OD means oculus dexter, or right eye. OS means oculus sinister, or left eye. You may also see OU, meaning both eyes. Some Korean optical forms write right and left in Korean or simply use R and L.
Do not assume the right and left numbers should be identical. It is common for the two eyes to have different focusing powers. The prescription must be made for the correct eye, and the optical shop should confirm this before making the lenses.
The main prescription fields
| Field | What it means | What the sign or units tell you | What it does not tell you |
|---|---|---|---|
| SPH or Sphere | Main lens power for near- or farsightedness | Minus usually corrects myopia; plus usually corrects hyperopia. Measured in diopters (D). | It does not describe the health of the cornea, optic nerve or retina. |
| CYL or Cylinder | Amount of astigmatism correction | May be written with a plus or minus sign, depending on the notation used. | It does not, by itself, explain why astigmatism is present. |
| AXIS | Orientation of the cylinder correction | A number from 1 to 180 degrees. It is an angle, not a power. | A changing axis is not automatically a sign of eye disease. |
| ADD | Extra plus power for reading or other near tasks | Usually a plus value, used in bifocals, progressives or other multifocal designs. | It does not mean the eye is “getting weaker” in a dangerous way. |
| PD | Pupillary distance; the distance between the pupils | Usually measured in millimetres; it may be one binocular number or two monocular numbers. | It is not a substitute for an accurate refraction or eye-health check. |
What does SPH mean?
SPH is the spherical lens power. It is the same in every direction across the lens. A minus value, such as −2.00 D, usually means the eye is myopic: without correction, distant objects are less clear. A plus value, such as +1.50 D, usually means hyperopia: the eye needs plus power for some focusing tasks.
The size of the number indicates the amount of optical power, but it is not a score of eye health. A person with a small prescription can still have dry eye, glaucoma, cataract or retinal disease. A person with a high prescription may have healthy eyes apart from the refractive error.
Small changes in SPH can be meaningful for some people and barely noticeable for others. The best prescription is the clearest, most comfortable correction that matches the refraction, binocular vision and daily needs—not necessarily the strongest number.
What do CYL and AXIS mean?
CYL corrects astigmatism, which occurs when the eye or cornea has different focusing power in different meridians. Astigmatism can cause blur, ghosting, glare or eye strain at distance and near. It can occur together with myopia or hyperopia.
AXIS tells the laboratory where to place the cylinder correction. It is written from 1 to 180 degrees. An axis of 90 is not “stronger” than an axis of 180; it is simply a different orientation.
You may see the same optical prescription written in plus-cylinder or minus-cylinder form. The numbers can look different while describing the same lens. If you compare two prescriptions, ask the clinician or optician whether they have been transposed before assuming your eyes changed.
What is ADD, and why does it appear after age 40?
ADD means additional near power. It is commonly prescribed when the natural lens loses some ability to change focus from far to near, a normal age-related process called presbyopia. The ADD is used in reading glasses, bifocals, progressives and some occupational lenses.
For example, a distance prescription might show SPH −1.00 and ADD +1.50. That does not mean the distance power becomes +0.50 everywhere. It means the near portion of a multifocal lens receives additional plus power. The final design also depends on the cylinder, axis, fitting height, working distance and frame.
Presbyopia can coexist with cataract, dry eye or other conditions. If reading vision changes quickly, one eye becomes noticeably different, or you have glare, double vision or distortion, do not simply increase the ADD without an eye examination.
What is PD, and why does it matter?
PD is pupillary distance. It helps place the optical centre of each lens in front of the pupil. Some prescriptions give one binocular PD, while others list a right and left monocular PD, such as 31/30 mm.
PD is especially important for higher prescriptions, progressive lenses and lenses with strong astigmatism. A small measurement error may cause discomfort, pulling or blur, but PD itself does not diagnose a problem. The glasses should be checked on your face, because frame shape, bridge position and how the frame sits can affect the final optical alignment.
What do VA, BC, DIA and other contact-lens fields mean?
VA usually means visual acuity—the smallest line you can read on a vision chart with or without correction. It is a measurement of performance at that visit, not a complete statement about eye health.
A contact-lens prescription has additional details, often including brand, base curve (BC), diameter (DIA), power and sometimes cylinder, axis or multifocal parameters. Contact lenses sit on the cornea, so their power and fit cannot always be copied directly from glasses. Tear-film quality, corneal shape, oxygen transmission and lens movement all matter.
Never order a different brand or wear a friend’s lenses because the SPH number looks similar. A proper contact-lens fitting includes a health check, measurements, trial wear and follow-up. Our guide on contact lenses that feel dry by afternoon explains why fit and tear stability can change comfort.
Prescription versus eye-health examination
A refraction answers a focused question: which lens power makes the letters clearest? A comprehensive eye examination asks broader questions about the cornea, lens, pressure, optic nerve, retina, macula, eye movements and visual field when indicated.
Book a full eye examination rather than only updating glasses if you have new flashes or floaters, a curtain or shadow, eye pain, persistent redness, sudden vision loss, distorted lines, recurrent headaches, diabetes, high myopia, a family history of glaucoma, or a large change in one eye. New symptoms are not explained safely by changing SPH alone.
| Situation | What a glasses update may help | When an eye doctor should examine you |
|---|---|---|
| Gradual distance blur | Refraction and new single-vision lenses may improve clarity | If the change is one-sided, rapid or associated with distortion |
| Near blur after 40 | Reading power, occupational lenses or progressives may help | If glare, double vision, pain or sudden change is present |
| Burning or fluctuating blur after screen work | Updated prescription may help if it is outdated | If symptoms recur despite breaks; assess tear film, lids and glands |
| New flashes, floaters or a shadow | Glasses do not treat a retinal tear | Same-day dilated retinal assessment |
For office workers in Gasan and Guro, long near-work days can combine an uncorrected prescription with dry eye and reduced blinking. Our digital eye strain guide covers screen habits, tear-film symptoms and when an examination is useful.
Common mistakes when reading a prescription
- Thinking a stronger lens is always better: Over-correction can cause strain, blur or difficulty relaxing focus.
- Mixing up right and left: Check OD/OS or R/L before ordering.
- Treating axis as power: Axis is an orientation in degrees.
- Copying a glasses prescription into a contact-lens order: Contact lenses need their own fitting.
- Assuming an unchanged prescription means healthy eyes: Refraction does not rule out glaucoma, cataract or retinal disease.
- Using an old prescription after a new symptom: Symptoms such as a curtain, flashes or sudden loss of vision need clinical assessment.
Frequently asked questions
Is −2.00 a high prescription?
It is a moderate minus spherical value in many optical systems, but “high” depends on the whole prescription and eye anatomy. The number does not describe retinal or optic-nerve health.
Can CYL be written with a plus sign in Korea and a minus sign elsewhere?
Yes. Plus- and minus-cylinder notation can represent the same optical correction after transposition. Compare the full prescription, not one sign in isolation.
What does an axis of 180 mean?
It is the orientation of the astigmatism correction. It is not 180 diopters and is not a severity score.
Can I use my glasses prescription to buy contacts online?
No. A contact-lens prescription must specify a suitable lens and fit after an eye-health and contact-lens assessment.
Does ADD mean I have cataracts?
No. ADD usually reflects presbyopia. Cataract is clouding of the natural lens and needs a separate examination.
Why is my PD missing?
Some clinics provide PD separately or measure it at the optical shop. Ask for it before ordering, especially for progressives or strong prescriptions.
Why are my new glasses uncomfortable?
Possible reasons include adaptation, incorrect lens power, axis, PD, frame fit, lens design or an eye-surface problem. If discomfort persists, have both the glasses and your eyes checked.
How often should I update my glasses?
There is no universal interval. Update them when vision or comfort changes, or when an eye examination shows a meaningful change. Adults with risk factors may need regular health checks even when the prescription is stable.
English-speaking eye examinations in Seoul
At Nunehim Eye Center in Guro, Dr. Daniel Jinhag Baik provides fluent English and Korean consultations for international patients and local families. We can discuss a glasses prescription, contact-lens concerns, screen-related symptoms and the eye-health checks that may be appropriate.
Address: 551 Siheung-daero, Guro-gu, Seoul
Phone: +82 2 2039 9322
Hours: Mon–Fri 09:00–18:30 (lunch 13:00–14:00); Sat 09:00–14:00; Sun/holidays closed
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Sources
- American Academy of Ophthalmology: How to Read an Eyeglasses Prescription
- American Optometric Association: Understanding and using eyeglass prescriptions
This article explains common prescription fields and does not replace an in-person eye examination. If you have sudden pain, vision loss, flashes or a curtain-like shadow, seek urgent eye care.
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