{"id":1816,"date":"2026-10-20T08:53:07","date_gmt":"2026-10-20T08:53:07","guid":{"rendered":"https:\/\/laseristanbulclinic.com\/eye-glossary\/cylinder\/"},"modified":"2026-10-20T12:56:38","modified_gmt":"2026-10-20T12:56:38","slug":"cylinder","status":"publish","type":"page","link":"https:\/\/laseristanbulclinic.com\/en\/eye-glossary\/cylinder\/","title":{"rendered":"Cylinder (CYL): What It Means on Your Eye Prescription"},"content":{"rendered":"\n
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  1. Home<\/a><\/li>
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  4. Cylinder (CYL): What It Means on Your Eye Prescription<\/span><\/li><\/ol><\/div>\t\t\t

    Cylinder (CYL): What It Means on Your Eye Prescription<\/h1>\n\t\t\t\t\t\t

    Medically reviewed by {{surgeon}}<\/span><\/strong>, {{surgeon’s title and specialty}}<\/span>, on {{review date}}<\/span>. Last updated {{date}}<\/span>.<\/p>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

    \"Illustration<\/figure><\/div>\n\t\t\t<\/div><\/div>\n<\/section>\n\n\n\n\n
    \n\t\t<\/svg>Definition<\/span>\n\t\t\t

    Cylinder (CYL, also written Cyl)<\/strong> (noun)<\/em>: 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.<\/p>\n\t\t<\/div><\/div><\/div><\/div><\/section>\n\n\n\n\n

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    Cylinder (CYL): key facts<\/h2><\/div>\n\t\t\t\t\t<\/div>\n\t\t\t
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    <\/svg>What it corrects<\/span><\/dt>
    Astigmatism: an imperfect curve of the cornea (the clear front of the eye) or of the lens inside it<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>Unit<\/span><\/dt>
    Dioptres (D), usually in steps of 0.25<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>Written with<\/span><\/dt>
    An axis from 1 to 180 degrees, for example \u22121.00 \u00d7 180<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>Sign<\/span><\/dt>
    Plus (+) or minus (\u2212), depending on the notation used; both describe the same lens<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>Typical amount<\/span><\/dt>
    Most people have 0.5 to 0.75 D of astigmatism; 1.5 D or more typically needs correction<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>How common<\/span><\/dt>
    About 4 in 10 adults (40.4%) and about 15% of children worldwide have astigmatism<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>Treatment<\/span><\/dt>
    Glasses, toric contact lenses, laser eye surgery, toric implantable lens, toric lens replacement<\/dd><\/dl><\/div>\n\t\t\t\t
    <\/svg>Specialist<\/span><\/dt>
    Ophthalmologist; at Laser Istanbul Clinic, Prof. Dr. Ahmet Sat\u0131c\u0131 or Op. Dr. Aysan Bayram (cornea and keratoconus: Do\u00e7. Dr. Muhammet Derda \u00d6zer)<\/dd><\/dl><\/div>\n\t\t\t<\/div>\n\t\t<\/div><\/div><\/div><\/section>\n\n\n\n\n
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    What does the cylinder eye prescription number mean?<\/h2><\/div>

    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.<\/p><\/div><\/div>

    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.<\/p>\n

    Example:<\/strong> \u22122.00 \u22121.00 \u00d7 180 means a sphere of \u22122.00 D (short sight), a cylinder of \u22121.00 D and an axis of 180 degrees.<\/p>\n

    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.<\/p><\/div>

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    How much cylinder is normal, and how much is too much?<\/h3>

    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.<\/p>

    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.<\/p><\/div><\/div>

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    What if the CYL box is empty, or says DS?<\/h3>

    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.<\/p><\/div><\/div><\/div>\t<\/div><\/div><\/div>\n<\/section>\n\n\n\n\n

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    Cylinder and axis: how they work together<\/h2>\n\t\t\t\t

    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).<\/p>\n

    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 \u2192<\/a><\/p>\n

    With-the-rule, against-the-rule and oblique astigmatism<\/h3>\n

    EyeWiki, the AAO’s online encyclopaedia, groups regular astigmatism by the direction of the steepest curve of the cornea:<\/p>

    Type<\/th>Steepest curve of the cornea<\/th>Where it is most common<\/th><\/tr><\/thead>
    With-the-rule<\/strong><\/th>Vertical<\/td>Children and adolescents<\/td><\/tr>
    Against-the-rule<\/strong><\/th>Horizontal<\/td>The most common type in adults over 40<\/td><\/tr>
    Oblique<\/strong><\/th>Diagonal (neither vertical nor horizontal)<\/td>Less common<\/td><\/tr><\/tbody><\/table><\/div>

    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.<\/p><\/div>\t<\/div><\/div><\/div><\/section>\n\n\n\n\n

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    Plus cylinder and minus cylinder: the same lens written two ways<\/h2>\n\t\t\t

    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.<\/p>\n\t\t\t

    How to convert between them (transposition):<\/h3>
    1. Add the sphere and the cylinder to get the new sphere.<\/li>
    2. Change the sign of the cylinder.<\/li>
    3. Add 90 to the axis; if the result is more than 180, subtract 180.<\/li><\/ol><\/div>
      Minus-cylinder form<\/th>Plus-cylinder form (same lens)<\/th><\/tr><\/thead>
      \u22122.00 \u22121.00 \u00d7 180<\/th>\u22123.00 +1.00 \u00d7 90<\/td><\/tr>
      \u22122.25 \u22121.50 \u00d7 180<\/th>\u22123.75 +1.50 \u00d7 90<\/td><\/tr>
      +1.00 \u22122.00 \u00d7 180<\/th>\u22121.00 +2.00 \u00d7 90<\/td><\/tr><\/tbody><\/table><\/div>

      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.<\/p>\n

      Spherical equivalent.<\/strong> Eye care professionals sometimes summarise a prescription as one number, the spherical equivalent: the sphere plus half the cylinder. For \u22122.00 \u22121.00 \u00d7 180 it is \u22122.50 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.<\/p>\n

      Mixed astigmatism.<\/strong> 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 \u22122.00 \u00d7 180 is an example: the two powers are +1.00 and \u22121.00. This is called mixed astigmatism.<\/p><\/div>\t<\/div><\/div><\/div><\/section>\n\n\n\n\n

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      What causes cylinder (astigmatism)?<\/h2><\/div>

      Doctors do not know exactly why corneas and lenses vary in shape, according to the AAO. What is known:<\/p><\/div><\/div>\t\t\t\t