An eye prescription for astigmatism includes three specific values that describe the necessary lens power. Understanding the role of each number is the best way to demystify your vision correction. This guide explains why your specific axis number is an orientation measurement and not an indicator of the severity of your astigmatism.
Understanding Astigmatism and the Prescription Components
Astigmatism is a common condition resulting from an imperfect curvature of the cornea or the lens inside the eye. Instead of being uniformly rounded like a basketball, the eye surface is shaped more like a football, possessing two different curves. This irregularity causes light to refract unevenly, leading to blurred or distorted vision at any distance.
To correct astigmatism, a prescription lists three main components for each eye. The Sphere (SPH) indicates the lens power needed for nearsightedness or farsightedness. The Cylinder (CYL) measures the amount of lens power required to correct the astigmatism, expressed in diopters. The Axis (AXIS) directs the precise placement of the astigmatism correction and is only present if a cylinder value exists.
What the Axis Number Represents
The Axis number is a measurement of orientation, not of power or severity. It is defined on a scale from 1 to 180 degrees, which helps pinpoint the exact location of the astigmatism on the eye’s surface. You can imagine the eye like a compass, where 90 degrees represents the vertical meridian and 180 degrees represents the horizontal meridian.
The purpose of the axis value is to tell the lens manufacturer where to align the corrective cylinder power. Without this precise angular measurement, the corrective lens would be rotated incorrectly, resulting in ineffective vision correction. The Axis value simply defines the angle between the two main curves of the astigmatic eye.
Why 175 Axis is Not a Measure of Severity
The number 175 in your prescription is simply a coordinate, indicating a location near the horizontal plane (180 or 0 degrees). This number alone tells you nothing about the magnitude or severity of your astigmatism. For example, a prescription with a 175 axis and a low Cylinder value is mild, while the same axis with a high Cylinder value is considered more severe.
The severity of astigmatism is determined solely by the dioptric value in the Cylinder (CYL) column. A higher Cylinder number means the eye’s shape is more irregular, requiring a stronger corrective lens. The Axis number, whether it is 175, 90, or 45, is only a directional instruction for the lens maker and does not indicate a stronger or weaker prescription.
Correcting Vision with Astigmatism
The Axis measurement is crucial for ensuring the corrective lens works as intended. In corrective lenses, such as eyeglasses or specialized toric contact lenses, the lens power must be oriented to match the eye’s irregular curvature. For a 175 axis, the corrective power is placed very close to the horizontal line of the eye.
An axis of 175 degrees often falls within the range of “with-the-rule” astigmatism. This type is characterized by the steepest curve of the cornea being oriented vertically, meaning the corrective lens power is placed near the 180-degree or horizontal axis. Toric contact lenses are designed to prevent rotation, ensuring the corrective power stays precisely at the prescribed axis, such as 175 degrees, for continuous clear vision.