A contact lens prescription contains various numbers and abbreviations defining the lens specifications required for clear vision. These values often include the Sphere (PWR/SPH), which corrects nearsightedness or farsightedness, and the Base Curve (BC) and Diameter (DIA), which ensure proper lens fit. For many patients, the prescription also contains a specific measurement labeled “Cylinder” (CYL). This measurement indicates a common refractive error requiring a specialized lens design and is always paired with two other related numbers to complete the correction.
The Visual Problem Astigmatism
The presence of a cylinder value indicates astigmatism, a common condition where the eye’s shape is irregular. Instead of a perfectly spherical front surface, the cornea or the lens inside the eye is shaped more like a football. This difference in curvature means the eye has two different curves at perpendicular angles, or meridians.
This non-spherical shape causes light entering the eye to bend unevenly. Light rays are focused onto the retina at two different points instead of converging to a single, sharp point. This results in blurry, distorted, or stretched vision at all distances, which can also lead to eye strain and headaches.
Decoding the Cylinder Measurement
The Cylinder (CYL) is the precise numerical value indicating the amount of extra power needed in the contact lens to correct astigmatism. Measured in diopters (D), the cylinder value represents the severity of the irregular curvature. This power is added to the main Sphere power to create a lens that corrects both simple refractive error and astigmatism simultaneously.
Astigmatism prescriptions are always written in the minus cylinder format, with values increasing in 0.25 diopter increments. A higher negative number signifies a greater degree of correction is required to achieve clear vision. Unlike the sphere power, this cylinder power is not applied uniformly across the entire lens surface; it is incorporated only along a specific angular orientation.
The Essential Partner: Axis
Because astigmatism involves an uneven curve, the corrective cylinder power must be paired with an “Axis” measurement. The Axis (AX) specifies the exact orientation on the eye where the power is needed. This figure is a number between 1 and 180 degrees that pinpoints the specific meridian where the maximum astigmatism occurs.
If the axis is 90 degrees, for example, the astigmatism is oriented vertically, while 180 degrees indicates a horizontal orientation. The axis value describes the direction of the uneven curve but does not relate to the strength of the prescription. Without this precise angular orientation, the cylinder power would be misaligned, leading to blurry or tilted vision.
How Toric Lenses Maintain Alignment
Lenses that contain both sphere and cylinder correction are known as toric contact lenses. They require specialized engineering to ensure the correction stays aligned on the specific axis. Since the lens sits on the eye and can rotate with every blink, manufacturers must build in stabilization techniques. These techniques keep the cylinder power in the correct orientation by subtly interacting with the eyelids and the tear film to maintain rotational stability.
Prism Ballasting
This common method involves making the bottom edge of the lens slightly thicker or heavier by incorporating a base-down prism. This extra mass uses gravity and the pressure of the lower eyelid to guide the lens to a consistent, stable position.
Dynamic Stabilization
This technique involves creating thin zones at the top and bottom of the lens. When the wearer blinks, the eyelids apply pressure to these thin zones, gently rotating the lens back into its intended alignment.