An eye prescription translates the optical needs of your eyes into specifications for corrective lenses. It is a standardized formula used by eye doctors worldwide to ensure you receive the proper vision correction. Understanding the core terms—Sphere, Cylinder, and Axis—demystifies the process of getting new glasses or contact lenses. These three components describe the exact power, shape, and orientation required for your corrective lens.
Understanding the Sphere (SPH) Value
The Sphere (SPH) is the foundational measurement of lens power, addressing the most common vision issues. This number indicates the uniform power applied across the entire lens surface to correct for either nearsightedness or farsightedness. The value is measured in diopters, and the number’s distance from zero signifies the strength of the correction required.
The sign preceding the SPH value indicates the specific type of refractive error being corrected. A minus sign \((-)\) means the correction is for myopia (nearsightedness), where distant objects appear blurry. Conversely, a plus sign \((+)\) or the absence of a sign indicates correction for hyperopia (farsightedness), which causes difficulty focusing on objects up close. If the SPH column lists “0” or “PL” (plano), no spherical correction is necessary for that eye.
Defining Cylinder (CYL) and Axis (AXIS)
When the eye’s shape is not perfectly spherical, but instead is curved more like a football, the condition is known as astigmatism. Astigmatism causes light to focus at multiple points instead of a single point on the retina, leading to blurry or distorted vision. The Cylinder (CYL) and Axis (AXIS) values are the two components that correct for this uneven curvature.
The Cylinder (CYL) value specifies the amount of extra lens power needed to compensate for the astigmatism. Like the sphere, this measurement is given in diopters, and a higher number indicates a greater degree of astigmatism. If the CYL column is blank or contains a zero, it means you have no astigmatism or the amount is too minor to warrant correction.
The Axis (AXIS) value is the orientation or angle at which the cylinder correction must be applied to the lens. It is represented by a number between 1 and 180 degrees, which tells the lab where to position the cylindrical power to align with the irregular curve of your eye. For example, 90 degrees corresponds to a vertical orientation, while 180 degrees indicates a horizontal orientation. A CYL value will always be accompanied by an AXIS value because the cylinder power must be precisely aligned.
Interpreting Your Complete Lens Correction
The combination of the Sphere, Cylinder, and Axis values dictates the shape of your corrective lens. A prescription that only contains a SPH value results in a simple, uniformly curved lens, known as a spherical lens. This lens provides the same power across all meridians to correct basic nearsightedness or farsightedness.
If your prescription includes values for SPH, CYL, and AXIS, the resulting lens is a more complex shape known as a toric lens. This lens has two different curvatures, where the power gradually changes across the surface to correct both the spherical error and the astigmatism in the direction specified by the Axis. These values are determined separately for each eye, often abbreviated as OD (right eye) and OS (left eye), because vision needs are rarely identical. The prescription ensures light focuses exactly where it needs to be for clear vision.