When reviewing a contact lens prescription, you may encounter terms that appear complex, like “CYL” for cylinder. This measurement is included only when a specific type of vision correction is required to address astigmatism. The cylinder value represents the precise power needed for correction. This article explains the purpose of the cylinder measurement and how it translates to specialized contact lenses.
What the Cylinder Measurement Addresses
The presence of a cylinder measurement indicates the wearer has astigmatism, a common type of refractive error. Astigmatism is an imperfection in the eye’s curvature that causes distorted or blurred vision at any distance. In a perfectly shaped eye, the cornea is spherical, but with astigmatism, the cornea is shaped more like a football. This means one meridian is steeper than the one perpendicular to it.
This irregular curvature causes light entering the eye to focus at two different points instead of a single, precise point. The result is vision that appears stretched or shadowed, as the eye is unable to form a sharp image. Astigmatism can occur in the cornea (corneal astigmatism) or in the lens inside the eye (lenticular astigmatism). The cylinder measurement on a prescription is the precise corrective power needed to counteract this irregular shape. Without this specific correction, a standard spherical contact lens would fail to provide clear vision.
Decoding the Cylinder and Axis Values
The cylinder, abbreviated as “CYL,” specifies the magnitude of the extra focusing power required to neutralize astigmatism. This value is always negative on a prescription, measured in diopters (e.g., -0.75 or -1.50), and increases in increments of 0.25. The higher the numerical value, regardless of the negative sign, the greater the degree of astigmatism that needs correction. The cylinder value is always paired with a second, equally important measurement called the Axis, abbreviated as “AX” or “AXIS.”
The Axis is a number between 1 and 180 degrees that defines the exact orientation of the astigmatism on the eye. This pairing provides the specific geometric information necessary to manufacture the correct lens. If the Axis is incorrect by even a few degrees, the corrective power will not align with the eye’s curvature, leading to blurred or fluctuating vision. This combination differentiates an astigmatism-correcting prescription from a standard one, which only lists spherical power (SPH or PWR).
Toric Lenses Correcting Astigmatism
Contact lenses that contain a cylinder measurement are known as toric lenses, named for their specialized geometric design. Unlike standard spherical lenses, which have a single curve and uniform power, toric lenses have two different power curves. One curve corrects for basic nearsightedness or farsightedness, while the second, cylindrical curve addresses the astigmatism.
For a toric lens to provide clear vision, it must remain precisely positioned on the eye, aligning the cylinder power with the prescribed Axis. Since contact lenses naturally rotate with blinks and eye movements, toric lenses incorporate stabilization features to maintain their orientation. These stabilization methods are engineered into the lens design to interact with the eyelids.
Stabilization Techniques
Common stabilization techniques include prism ballast, where the bottom edge of the lens is slightly thicker and heavier, leveraging eyelid pressure to keep it upright. Another approach is dynamic stabilization, which uses thin zones on the top and bottom edges of the lens. These sophisticated designs ensure the cylinder correction remains properly aligned with the eye’s irregular curvature, providing consistent and sharp vision.