An eye prescription, often referred to as an Rx, is a standardized set of measurements used to correct common vision problems called refractive errors. These errors occur when the eye fails to focus light precisely onto the retina, leading to blurry vision. The numbers and letters on your prescription define exactly how a lens needs to bend light to achieve clear sight. Understanding these measurements is the first step toward knowing how your specific vision correction works.
What is Astigmatism
The need for an axis measurement on a prescription is directly related to astigmatism, a refractive error. Astigmatism is an imperfection in the curvature of the eye’s cornea, the clear front surface, or the lens inside the eye. Instead of having a perfectly spherical shape, the affected surface is curved more like an oval.
This irregular shape causes light entering the eye to focus on multiple points instead of a single, sharp point on the retina. As a result, vision becomes blurred or distorted at all distances. Astigmatism frequently occurs alongside nearsightedness or farsightedness.
The Role of Cylinder and Axis
Correcting astigmatism requires a specific lens element, which is why your prescription will contain both a Cylinder (CYL) and an Axis (AX) value. The Cylinder number specifies the amount of lens power needed to correct the irregularity in the eye’s curvature. This value is measured in diopters, similar to the Sphere (SPH) value used for general nearsightedness or farsightedness correction.
The Axis number, a degree between 1 and 180, acts as the instruction for where that cylindrical power must be applied. Astigmatism only affects vision along a certain angle or meridian of the eye, not uniformly across the entire surface. Therefore, the cylindrical lens power must be aligned exactly to counteract the specific orientation of the eye’s irregular curve.
The Axis value is meaningless without an accompanying Cylinder value, as it simply describes the angle of the correction. The Cylinder is the strength of the correction and the Axis is the location on the lens where that strength is concentrated. Getting the Axis correct is crucial, because misalignment can lead to blurred vision and eye strain.
Interpreting the Orientation Scale
The Axis is measured on an angular scale from 1 to 180 degrees, mapping the orientation around the center of your eye. This scale describes the eye’s meridians, or imaginary lines running across the eye’s surface. The 90-degree mark represents the vertical meridian, and the 180-degree mark represents the horizontal meridian.
A prescription with an axis of 170 degrees means the required cylindrical power is oriented very close to the horizontal plane. The correction is applied 10 degrees away from the exact horizontal meridian of 180 degrees. This indicates that the steepest or flattest part of the astigmatism is aligned nearly horizontally. The number itself does not indicate the severity of the astigmatism, only its specific rotational alignment.
This precise degree value is an instruction for the optical lab to grind the lens and ensure the cylindrical correction is placed at that exact angle. If the lens is rotated even slightly, the correction will not align with the irregularity on your cornea, resulting in distorted vision.