The combination of letters and numbers on an eyeglass or contact lens prescription represents precise measurements required to correct specific vision errors. Understanding the terms Sphere (SPH) and Plano (PL) is a fundamental first step in decoding a vision prescription. These values specify the main corrective power needed for each eye, setting the stage for the lens design that will bring your vision into focus.
Defining Sphere (SPH) and Plano (PL)
The term Sphere (SPH) indicates the main lens power required to correct overall refractive errors, such as nearsightedness or farsightedness. These errors are uniform across all meridians of the eye. The term “sphere” is used because the required correction is the same in every direction across the lens.
This value represents a uniform curvature ground into the lens surface to properly redirect light onto the retina. If there is no number or a zero is listed, the abbreviation PL (Plano) is used. Plano signifies a lens surface with zero optical power.
Seeing PL or Plano under the SPH heading indicates that no correction is necessary for distance vision in that eye. This means the eye’s natural focusing ability is considered normal for that refractive error. For patients who only require correction for astigmatism, the SPH value may also be listed as Plano.
Understanding the Diopter Value (Plus and Minus Signs)
The numerical value listed under SPH is measured in diopters (D), the international unit for measuring the optical power of a lens. This value quantifies the degree of refractive error, indicating how strong the corrective lens must be. The further the absolute number is from zero, the stronger the prescription and the greater the correction required.
The sign preceding the diopter value defines the type of refractive error being corrected. A minus sign (-) indicates nearsightedness (myopia). In myopia, the eye focuses light in front of the retina, and the corrective lens must be concave to diverge the light rays so they land directly on the retina.
Conversely, a plus sign (+) or the absence of a sign indicates farsightedness, or hyperopia. Hyperopia occurs when the eye focuses light behind the retina, requiring a convex corrective lens that converges, or brings the light rays together, to focus them properly. For instance, a prescription of +2.50 D corrects a moderate degree of farsightedness, while a prescription of -4.00 D corrects a higher degree of nearsightedness.
Distinguishing Sphere from Cylinder (CYL)
The Sphere value specifically addresses refractive errors that result from an overall mismatch between the eye’s length and its focusing power. These errors are spherical, meaning the eye’s curvature is perfectly round. The SPH value provides a single, uniform power across the entire lens surface to correct this type of error.
The Cylinder (CYL) value is a separate measurement that corrects for a condition called astigmatism. Astigmatism is a non-uniform refractive error where the cornea or lens is shaped more like a football, with different curvatures in different directions. This irregularity causes light to focus at multiple points instead of a single point, leading to blurred or distorted vision.
The CYL value indicates the amount of additional lens power required to correct this uneven curvature. Because the power is not uniform, any prescription with a CYL value must also include an Axis value, which specifies the orientation or angle at which the cylindrical correction must be placed on the lens. The SPH value is distinct because it handles the basic, uniform focus of the eye, while the CYL value refines the focus by correcting for non-uniform irregularities.