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Cylinder (CYL): What It Means on Your Eye Prescription
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Cylinder (CYL, also written Cyl) (noun): the cylinder number in a glasses prescription is the extra lens power that corrects astigmatism, an eye shape that is more rugby ball than football, so that light focuses in more than one place. It is measured in dioptres (D) and always comes with an axis, a number from 1 to 180 that says in which direction the extra power is placed. If the CYL column is empty, that eye has no astigmatism that needs correcting.
Cylinder (CYL): key facts
- What it corrects
- Astigmatism: an imperfect curve of the cornea (the clear front of the eye) or of the lens inside it
- Unit
- Dioptres (D), usually in steps of 0.25
- Written with
- An axis from 1 to 180 degrees, for example −1.00 × 180
- Sign
- Plus (+) or minus (−), depending on the notation used; both describe the same lens
- Typical amount
- Most people have 0.5 to 0.75 D of astigmatism; 1.5 D or more typically needs correction
- How common
- About 4 in 10 adults (40.4%) and about 15% of children worldwide have astigmatism
- Treatment
- Glasses, toric contact lenses, laser eye surgery, toric implantable lens, toric lens replacement
- Specialist
- Ophthalmologist; at Laser Istanbul Clinic, Prof. Dr. Ahmet Satıcı or Op. Dr. Aysan Bayram (cornea and keratoconus: Doç. Dr. Muhammet Derda Özer)
What does the cylinder eye prescription number mean?
The cylinder (CYL) number on your eye prescription tells you how much astigmatism your glasses need to correct. The American Academy of Ophthalmology (AAO) defines it as the number that shows how much astigmatism you have, if any.
A glasses prescription is read from left to right for each eye. The first number, the sphere (SPH), corrects short or long sight; the second, the cylinder (CYL), corrects astigmatism; the third, the axis, says where the cylinder is placed. The Royal College of Ophthalmologists (RCOphth) describes it the same way: the first number gives the amount of long or short sight, the second the amount of astigmatism. The right eye is usually listed as OD and the left as OS.
Example: −2.00 −1.00 × 180 means a sphere of −2.00 D (short sight), a cylinder of −1.00 D and an axis of 180 degrees.
A cylinder lens works differently from the sphere. A normal (spherical) lens focuses light equally in every direction. A cylindrical lens changes the focus in one direction only: it bends light perpendicular to its axis and leaves light parallel to its axis untouched. That is why the CYL number can never stand on its own, and why the axis always follows it.
How much cylinder is normal, and how much is too much?
Cylinder is measured in dioptres, a unit of lens power equal to one reciprocal metre: a 3 D lens focuses parallel light at one-third of a metre. Prescriptions go up in steps of 0.25 D.
The AAO says most people have between 0.5 and 0.75 D of astigmatism, and that astigmatism of 1.5 D or more typically needs correction. A number on a prescription is not "good" or "bad" in itself: whether a small cylinder is corrected depends on whether it blurs your vision or gives you symptoms. The AAO also notes that astigmatism can change gradually over time, especially as the eye grows or ages, so it may worsen, improve or stay stable.
What if the CYL box is empty, or says DS?
An empty CYL column, a dash or "DS" (dioptres sphere) means a spherical-only prescription: there is no astigmatism to correct in that eye. If you see a cylinder number but no axis, ask your optician to check the prescription, because the two belong together.
Cylinder and axis: how they work together
The axis is a number from 1 to 180 degrees that tells the lens maker which way to turn the cylinder. The AAO describes it as the number that shows where the astigmatism lies on the cornea. On the ophthalmic scale, 180 degrees is horizontal and 90 is vertical, and 0 and 180 are the same line. A value above 180 is wrapped round: 185 degrees, for example, is noted as 5 degrees (AAO).
Two eyes with the same CYL can need very different lenses if the axis differs, so both numbers matter when you order glasses or contact lenses. A short guide to the axis itself is on its own page. Glossary: Axis →
With-the-rule, against-the-rule and oblique astigmatism
EyeWiki, the AAO's online encyclopaedia, groups regular astigmatism by the direction of the steepest curve of the cornea:
| Type | Steepest curve of the cornea | Where it is most common |
|---|---|---|
| With-the-rule | Vertical | Children and adolescents |
| Against-the-rule | Horizontal | The most common type in adults over 40 |
| Oblique | Diagonal (neither vertical nor horizontal) | Less common |
Which axis number this gives on your prescription depends on the notation: a steepest curve that is vertical is written as an axis near 90 in plus-cylinder notation and near 180 in minus-cylinder notation.
Plus cylinder and minus cylinder: the same lens written two ways
Some prescriptions show the cylinder with a minus sign and others with a plus sign, but both can describe the identical lens. According to a reference summary on eyeglass prescriptions, plus-cylinder notation is mainly used by European practitioners and US ophthalmologists, and minus-cylinder notation by US optometrists and many Asian practitioners. Both styles are in use, so you may be handed two different-looking prescriptions for the same eyes.
How to convert between them (transposition):
- Add the sphere and the cylinder to get the new sphere.
- Change the sign of the cylinder.
- Add 90 to the axis; if the result is more than 180, subtract 180.
| Minus-cylinder form | Plus-cylinder form (same lens) |
|---|---|
| −2.00 −1.00 × 180 | −3.00 +1.00 × 90 |
| −2.25 −1.50 × 180 | −3.75 +1.50 × 90 |
| +1.00 −2.00 × 180 | −1.00 +2.00 × 90 |
The second row is the worked example given in the reference summary; the other two rows are our own calculation using the three steps above.
Spherical equivalent. Eye care professionals sometimes summarise a prescription as one number, the spherical equivalent: the sphere plus half the cylinder. For −2.00 −1.00 × 180 it is −2.50 D. AAO refraction guidance shows how sphere and cylinder trade off: when a clinician adds +0.50 D of cylinder, they take away 0.25 D of sphere, so that the overall focus stays the same.
Mixed astigmatism. If the sphere and the sphere-plus-cylinder have opposite signs, one direction of the eye is long-sighted and the other short-sighted. +1.00 −2.00 × 180 is an example: the two powers are +1.00 and −1.00. This is called mixed astigmatism.
What causes cylinder (astigmatism)?
Doctors do not know exactly why corneas and lenses vary in shape, according to the AAO. What is known:
- Inheritance. The tendency to have astigmatism runs in families (AAO).
- Development. The National Eye Institute (NEI) says astigmatism can be present from birth or develop in childhood or young adulthood. In the first months of life it is very common: in one study about 55% of infants younger than 5 months had at least 1 D, and it falls markedly by age 18 to 36 months as the cornea changes (EyeWiki).
- The cornea does most of the work. EyeWiki states that the shape of the cornea accounts for the major part of total astigmatism; the lens inside the eye adds a smaller part.
- Eye disease, injury or surgery. Astigmatism can also appear after an eye injury, an eye disease or eye surgery (AAO, NEI).
- Keratoconus. In this condition the cornea thins and bulges into a cone, which causes astigmatism and a prescription that needs changing often (AAO). The NHS also links astigmatism with keratoconus, which is why regular eye tests matter.
- Age. In adults over 40, against-the-rule astigmatism is the most common type (EyeWiki).
Myths. Reading in dim light or sitting close to a screen does not cause astigmatism (AAO). The NEI says it cannot be prevented.
Regular and irregular astigmatism
In regular astigmatism the two main curves of the cornea sit at right angles to each other, and a cylinder lens can correct it. In irregular astigmatism the curves are uneven, often because of keratoconus, scarring or earlier surgery. EyeWiki states that irregular astigmatism cannot be fully corrected by cylindrical lenses. This is why a high or fast-changing cylinder deserves a corneal scan, not just new glasses.
Symptoms of uncorrected cylinder
Mild astigmatism may cause no symptoms (NEI). When the cylinder is larger, or your glasses are out of date, the AAO, NEI and NHS list these signs:
| Symptom | Listed by |
|---|---|
| Blurred or distorted vision | AAO, NEI, NHS |
| Eye strain, especially after long screen time | NEI, NHS |
| Headaches | AAO, NEI, NHS |
| Squinting | AAO, NEI |
| Difficulty seeing at night | NEI |
| Eye discomfort | AAO |
Children may not realise that their vision is blurred. The AAO warns that untreated astigmatism in children can lead to amblyopia (lazy eye), which is why vision screening for children matters.
How is cylinder measured?
Cylinder is measured at an eye examination, by an optometrist (optician) or an ophthalmologist. The AAO lists the main tools:
- Eye chart (visual acuity). Shows how clearly you see at a distance.
- Autorefractor. A machine that measures the refractive error of each eye.
- Phoropter. The binocular-like device with many lenses, in which you compare "better one or two?". It is used to determine the strength of your prescription, including the cylinder and axis.
- Keratometer. Measures the curvature of the cornea.
For anyone considering surgery, an eye surgeon also maps the cornea in detail (corneal topography) to check whether the astigmatism is regular or irregular. At Laser Istanbul Clinic, corneal topography, OCT and biometry are part of every pre-operative examination in Istanbul, and your surgeon reviews your existing prescription and scans before you book.
How is cylinder (astigmatism) treated?
The NHS notes that treatment is not always needed. When it is, the options are:
| Option | What it does | Notes |
|---|---|---|
| Glasses | Add a cylinder lens at the right axis | The most affordable option (NHS) |
| Toric soft contact lenses | Contact lenses with a built-in cylinder | For astigmatism; one of the two main types of contact lens for it (AAO) |
| Rigid gas-permeable (RGP) contact lenses | A firm lens that gives a smooth new front surface | The AAO says they may work best for more severe or irregular astigmatism |
| Laser eye surgery (LASIK, SMILE, PRK) | Reshapes the cornea, usually in the same treatment as short or long sight | The RCOphth says you are most likely to be suitable up to ±6.00 D of astigmatism |
| Toric ICL (implantable lens) | A lens with built-in astigmatism correction placed inside the eye | In the US it is FDA-approved for 1.0 to 4.0 D of cylinder (EyeWiki) |
| Toric lens replacement (cataract or refractive lens exchange) | The natural lens is replaced with a toric artificial lens | Toric lenses correct about 0.75 to 4.75 D of astigmatism at the corneal plane (EyeWiki) |
Limbal relaxing incisions (small cuts at the edge of the cornea) can be used at the time of cataract surgery; EyeWiki says they can correct up to 3.0 D but are most predictable up to 1.5 D.
Three points worth knowing:
- Alignment matters. For a toric implant, EyeWiki notes that every degree the lens is rotated off axis reduces its power by 3.3%. This is why the axis has to be measured precisely.
- Laser surgery is for stable prescriptions. The RCOphth says short sight and astigmatism normally stabilise in the late teens or early twenties, that natural changes can occur at any stage of life, and that laser sometimes needs to be repeated. You are most likely to be suitable if you are over 18 and your prescription has not changed by more than 0.5 D in the last two years.
- Irregular astigmatism is not a standard laser case. If your scans show keratoconus or a changing cornea, our cornea specialist, Doç. Dr. Muhammet Derda Özer ("Doç." is the Turkish title for Associate Professor), reviews your eyes first.
Treatment with us: Prof. Dr. Ahmet Satıcı and Op. Dr. Aysan Bayram treat astigmatism with laser eye surgery and, when your age or a cataract makes it the better option, with a toric lens. Prof. Dr. Satıcı also offers ICL. Your surgeon recommends one option only after measuring your eyes, and you can send your current prescription for a free review first.
When to see a doctor about your cylinder
See an optician or eye doctor soon if:
- you have blurred or distorted vision, headaches or eye strain, especially after reading or screen use
- your prescription has changed noticeably since your last test; the NHS advises an eye test if you have symptoms or have not had one for two years
- your child squints, sits close to the screen or says that things look blurry
- your cylinder keeps increasing year after year, especially if you are a teenager or young adult; ask whether a corneal scan is needed to rule out keratoconus
Seek same-day eye care for a sudden drop in vision, eye pain or an eye injury. These are not explained by astigmatism.
If you wear glasses and wonder whether surgery could free you of them, bring or send your full prescription, including the CYL and axis for each eye.
Frequently asked questions
CYL is the cylinder: the amount of lens power needed to correct astigmatism. It is measured in dioptres and always comes with an axis, which says in which direction the power is placed. An empty CYL column means no astigmatism needs correcting in that eye.
The cylinder is how much astigmatism you have, and the axis, a number from 1 to 180, shows in which direction the correcting lens power is placed. Both numbers are needed to make your lens, so they should always appear together.
It is a moderate amount of astigmatism, and whether it needs correcting depends on whether it blurs your vision. The AAO says most people have 0.5 to 0.75 D, and that 1.5 D or more typically needs correction. A −1.00 cylinder lies in between, so symptoms such as blur, headaches or eye strain are the practical test.
Nothing in the lens itself: they are two ways of writing the same prescription. To convert, add sphere and cylinder, change the sign of the cylinder and add 90 to the axis (subtract 180 if the result is over 180). So −2.00 −1.00 × 180 equals −3.00 +1.00 × 90.
Not necessarily. It means more astigmatism, which glasses or contact lenses correct in most people. A very high or fast-changing cylinder can point to keratoconus or another corneal problem, so it should be checked with a corneal scan.
Yes. The AAO says astigmatism can change gradually over time, especially as the eye grows or ages. The RCOphth adds that it normally stabilises in the late teens or early twenties, but natural changes can occur at any stage of life.
Yes, for many people. Laser reshapes the cornea to correct astigmatism together with short or long sight. The RCOphth says you are most likely to be suitable up to ±6.00 D of astigmatism, and a toric ICL or toric lens is an option for other cases. Your surgeon decides after measuring your eyes.
In practice, yes. The CYL number is the measured amount of astigmatism in dioptres, and the axis shows its direction.
Sources for the facts on this page
- American Academy of Ophthalmology. How to Read Your Eyeglasses Prescription (sphere, cylinder, axis, 1–180). https://www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription
- American Academy of Ophthalmology. What Is Astigmatism? (definition, causes, myths, symptoms, tests, treatment, 0.5–0.75 D and 1.5 D, change over time, amblyopia). https://www.aao.org/eye-health/diseases/what-is-astigmatism
- NHS. Astigmatism (rugby ball shape, symptoms, treatment options, eye test advice, keratoconus link). https://www.nhs.uk/conditions/astigmatism/
- National Eye Institute (NIH). Astigmatism (present from birth or develops, injury or surgery, cannot be prevented, symptoms). https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/astigmatism
- EyeWiki (AAO). Physiology of Astigmatism (with-the-rule, against-the-rule, oblique; infants; corneal share). https://eyewiki.org/Physiology_of_Astigmatism
- EyeWiki (AAO). Astigmatism Correction at the Time of Cataract Surgery (toric IOL range, limbal relaxing incisions, 3.3% per degree, irregular astigmatism). https://eyewiki.org/Astigmatism_Correction_at_the_Time_of_Cataract_Surgery
- EyeWiki (AAO). Implantable Collamer Lens (toric ICL FDA range 1.0–4.0 D). https://eyewiki.org/Implantable_Collamer_Lens
- American Academy of Ophthalmology, Young Ophthalmologists. Refraction 101: Go Forth and Refract (axis scale 1–180; sphere and cylinder trade-off). https://www.aao.org/young-ophthalmologists/yo-info/article/refraction-101-go-forth-and-refract
- American Academy of Ophthalmology. What Is Keratoconus? (astigmatism, frequent new prescriptions). https://www.aao.org/eye-health/diseases/what-is-keratoconus
- American Academy of Ophthalmology. What Is Presbyopia? (noticeable shortly after age 40; separate from astigmatism). https://www.aao.org/eye-health/diseases/what-is-presbyopia
- The Royal College of Ophthalmologists. Laser vision correction: patient information (PROF/2024/499): up to ±6.00 D astigmatism, prescription stability, stabilises late teens or early 20s. https://www.rcophth.ac.uk/wp-content/uploads/2026/08/Laser-vision-correction-patient-information.pdf (landing page: https://www.rcophth.ac.uk/guidance-resources/laser-vision-correction/)
- Hashemi H, Fotouhi A, Yekta A, et al. Global and regional estimates of prevalence of refractive errors: systematic review and meta-analysis. J Curr Ophthalmol 2018;30(1):3–22 (astigmatism 40.4% of adults, 14.9% of children). https://pubmed.ncbi.nlm.nih.gov/29564404/
- Wikipedia. Eyeglass prescription (plus and minus cylinder conventions, transposition rule, worked example, D.S.). Reference summary; the reviewing surgeon may confirm against a textbook. https://en.wikipedia.org/wiki/Eyeglass_prescription
- Wikipedia. Cylindrical lens (focuses perpendicular to its axis, leaves parallel light unaltered). Reference summary. https://en.wikipedia.org/wiki/Cylindrical_lens
- Wikipedia. Dioptre (one reciprocal metre; 3 D focuses at one-third of a metre; 0.25 D steps). Reference summary. https://en.wikipedia.org/wiki/Dioptre
Want to know whether your cylinder can be corrected without glasses?
Send us your glasses prescription, including the CYL and axis, and any eye reports. Prof. Dr. Ahmet Satıcı or Op. Dr. Aysan Bayram will tell you whether laser, an implantable lens or a toric lens suits your eyes, with no obligation.
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