An eyeglass prescription (Rx sheet) maps the specific lens power needed to correct a person’s vision. Seeing a mix of positive (+) and negative (-) numbers often confuses patients trying to decipher their correction. The combination of a positive sphere and a negative cylinder is not an error but a common, necessary configuration that addresses specific visual issues. This blend of powers creates a single, precisely shaped lens designed to bring light into sharp focus on the retina.
Defining Spherical Correction (Sphere)
The Sphere (SPH) value on a prescription indicates the power needed to correct the eye’s overall focus, assuming the eye’s front surface is perfectly round. This measurement addresses the primary refractive error, which is the uniform bending of light across the entire lens surface. The unit of measurement for this power is the diopter.
A positive (+) sign signifies farsightedness (hyperopia). This convex lens is thicker in the center and converges light rays that would naturally focus behind the retina. Conversely, a negative (-) sign indicates nearsightedness (myopia) and requires a concave lens that is thinner in the center. This lens diverges light rays that would otherwise focus in front of the retina.
Defining Astigmatism Correction (Cylinder and Axis)
The Cylinder (CYL) and Axis values are specifically dedicated to correcting astigmatism, a refractive error caused by the eye’s shape being more like a football than a perfect sphere. This irregularity causes light to focus unevenly, resulting in blurry or distorted vision. The cylinder value quantifies the extra lens power required to compensate for this unevenness.
The cylinder power is concentrated along a specific meridian, indicated by the Axis value. The axis is an angular measurement (1 to 180 degrees) that tells the lens manufacturer where to orient the correction. The cylinder power corrects the difference in focus between the two principal meridians of the eye. While cylinder power can be written as either positive or negative, the negative cylinder notation is the prevailing standard used by most eye care professionals in the United States.
How Positive Sphere and Negative Cylinder Work Together
The combination of a positive sphere and a negative cylinder is required when an eye is simultaneously farsighted and astigmatic. The final lens is a single, continuous surface, yet it has two distinct powers along its surface, separated by 90 degrees, making it a toric lens. This configuration most often corrects Compound Hyperopic Astigmatism or Mixed Astigmatism.
In this scenario, the positive sphere dictates the necessary converging power for the eye’s overall farsightedness. The negative cylinder introduces a diverging, or power-reducing, effect along one specific meridian defined by the axis. Mathematically, the total power along one principal meridian is the sum of the positive sphere and the negative cylinder, while the power 90 degrees away remains only the sphere power.
For instance, a prescription of +2.00 SPH with a -1.00 CYL at 90 degrees results in a lens with +2.00 D power along the 180-degree meridian. Along the 90-degree meridian, the total power is the sum of the sphere and cylinder (+2.00 D plus -1.00 D), equaling +1.00 D. The resulting lens corrects an eye that is farsighted but unequally so, requiring less converging power along one axis than the other. This combination allows the lens to correct the general focus while simultaneously correcting the specific irregularity of the astigmatism.