Eyeglass prescriptions contain three main values that define vision correction: Sphere (SPH), Cylinder (CYL), and Axis. The Sphere corrects for general nearsightedness or farsightedness. The Cylinder and Axis work together to address astigmatism, a specific refractive error. The Axis number, such as 175, is a precise measurement crucial for aligning the corrective power in your lenses.
The Role of Cylinder Power in Vision Correction
The Cylinder (CYL) value on your prescription indicates astigmatism, a common refractive error. Astigmatism occurs when the front surface of the eye, the cornea, or the lens inside the eye, has an imperfect, unequal curvature. Instead of being shaped like a perfect sphere, the eye’s curvature is more like a football, with one meridian being steeper than the perpendicular one.
This irregular shape causes light to focus on multiple points, rather than a single point on the retina, resulting in blurred or distorted vision. The Cylinder power addresses this issue by specifying the amount of extra lens power required to compensate for the difference in curvature. Measured in diopters (D), the Cylinder value indicates the strength of the correction needed.
A higher Cylinder number, such as -2.00, signifies a greater degree of astigmatism and requires stronger corrective power in the lens. This cylindrical power is ground into the lens to neutralize the uneven focus. The number tells the lens manufacturer how much power to add, but the Axis specifies where to place it.
What the Axis Measurement Represents
The Axis measurement dictates the precise orientation of the cylindrical power on your lens. While the Cylinder value provides the strength of the correction, the Axis specifies the exact angle at which that correction must be positioned. This angular measurement uses a protractor-like scale superimposed over the front surface of the eye.
This scale ranges from 1 to 180 degrees, with 90 degrees representing the vertical meridian and 180 degrees representing the horizontal meridian. The Axis number maps the location of the astigmatic curve. The corrective lens must be aligned perfectly with the eye’s unique orientation to be effective.
If the Axis is not aligned correctly, the cylindrical power will be placed in the wrong direction, leading to blurred vision or eye strain. The Axis number is purely a directional coordinate and does not relate to the strength of the prescription. For instance, 90 degrees indicates vertical astigmatism, while 180 degrees means the astigmatism is horizontal.
Interpreting the Specific 175 Axis Value
An Axis value of 175 is a specific directional setting for the cylindrical lens power. On the 1 to 180-degree scale, 175 is very close to the 180-degree mark, which represents the horizontal plane of the eye. This means the orientation of your astigmatism is nearly horizontal, tilted only five degrees.
The number 175 specifies the meridian where the cylindrical lens power is weakest. To achieve correction, the full cylindrical power is applied along the meridian perpendicular to the Axis. For a 175 Axis, the strongest corrective curve will be applied at 85 degrees (90 degrees away).
This precise angular placement ensures the lens is ground correctly to counteract the irregular curvature in the eye. An Axis of 175 is optically similar to an Axis of 5 degrees, as both are five degrees away from the horizontal 180/0 baseline. This ensures light is correctly focused onto the retina.