When you receive an eyeglass prescription, the string of abbreviations and numbers can often appear confusing. One of the most perplexing numbers is the one labeled “Axis,” especially when it falls on a high number like 175 degrees. This measurement is not an indication of prescription strength, but rather a precise instruction for how your corrective lens must be manufactured and aligned.
The Condition: What is Astigmatism?
The need for an Axis measurement is linked to astigmatism. This condition occurs when the cornea (the clear front surface of the eye) or the lens inside the eye is not perfectly spherical. Instead of a uniform, baseball-like curvature, the surface is shaped more like an oval.
This irregular shape means that light entering the eye is refracted differently along various meridians. Unlike a normal eye that focuses light to a single point on the retina, an astigmatic eye causes light to focus at multiple points. This uneven focusing results in blurry or distorted vision at all distances, often accompanied by eyestrain or headaches.
Eyeglass Prescription Basics: Sphere, Cylinder, and Axis
To correct vision, an eyeglass prescription contains three main components: Sphere, Cylinder, and Axis. The Sphere (SPH) value is the primary power needed to correct for nearsightedness (negative number) or farsightedness (positive number).
The Cylinder (CYL) value, measured in diopters, specifies the precise amount of extra lens power required to neutralize astigmatism. If this column is blank, it means astigmatism correction is not required. The Axis is always present if a Cylinder value is present, as it provides the necessary orientation for that cylindrical correction.
The Axis value is a measurement in degrees that tells the lens manufacturer the exact angle at which the Cylinder’s corrective power must be placed onto the lens. The Cylinder represents the strength of the correction, and the Axis provides the direction for applying that strength. Without the correct Axis, the cylindrical power would be placed incorrectly, leading to ineffective correction and visual distortion.
Interpreting the Axis: What the Degree Number Represents
The Axis measurement uses a scale ranging from 1 to 180 degrees, effectively mapping the curvature of the eye onto a semi-circle. This scale is visualized as a protractor laid over the eye. 90 degrees represents the vertical meridian, running from top to bottom. The 180-degree mark, which is equivalent to 0 degrees, represents the horizontal meridian.
An Axis of 175 degrees is very close to the horizontal meridian, meaning the irregular curvature of your eye is oriented nearly side-to-side. This falls into the category of a “with-the-rule” astigmatism, which is a common orientation where the vertical curve is steeper than the horizontal curve. A 175-degree Axis directs the lens maker to place the maximum astigmatism-correcting power almost horizontally on your lens.
This high degree number should not be mistaken for a measure of severity, as the Axis only describes the position, not the magnitude, of the astigmatism. Precision in this measurement is important because even a small misalignment of the cylindrical power can cause visual discomfort and decreased clarity. If the lens is rotated off the prescribed Axis, the patient might experience eye strain or have their vision appear clearest only through a small part of the lens.