The abbreviations and numbers on an eyeglass prescription can often look like a secret code. This prescription is an instruction sheet for the laboratory, ensuring the lenses precisely correct how the eye focuses light. Deciphering these terms is the first step toward understanding your visual health. The most fundamental measurement on this document is typically the one abbreviated as SPH.
Decoding Sphere (SPH)
The abbreviation SPH stands for “Sphere,” and it quantifies the overall refractive error of the eye, indicating the degree of nearsightedness or farsightedness. This measurement is the main component of the lens power, representing a uniform correction applied across the entire surface of the lens. It tells the optician how much the lens needs to bend light to focus sharply onto the retina.
The unit of measurement for SPH is the Diopter (D), which measures the optical power of a lens. A higher numerical value, regardless of the sign, signifies a stronger prescription. The SPH value is provided in increments of 0.25 D. For instance, a prescription of -4.00 D requires twice the lens power as a -2.00 D prescription.
If the SPH column lists “Plano” or “PL,” the eye requires no spherical lens power. The term “Sphere” refers to the light-bending power being equal in all meridians of the lens, like the surface of a round ball. This spherical design is sufficient when the eye’s shape is generally symmetrical, addressing the most common types of focusing errors.
Understanding the Plus and Minus Signs
The signs preceding the SPH value are significant because they identify the type of lens required and the underlying vision problem. A negative sign (-), is used for the correction of Myopia (nearsightedness). This condition means distant objects appear blurry because light focuses in front of the retina.
To correct nearsightedness, the prescription calls for a diverging, or concave, lens that is thinner in the center. This lens pushes the focal point backward, allowing the image to land on the retina. Conversely, a positive sign (+), signals Hyperopia (farsightedness). In this case, light tends to focus behind the retina, causing near objects to appear out of focus.
Farsightedness is corrected with a converging, or convex, lens that is thicker in the center. This lens pulls the focal point forward onto the retina. The sign is a direct instruction to the lens manufacturer about the shape and function of the lens needed.
Sphere Versus Other Prescription Measurements
While SPH addresses overall focusing power, prescriptions may include other terms for complex refractive errors. The Sphere measurement is distinct from the Cylinder (CYL) and Axis measurements, which correct for Astigmatism. Astigmatism is an optical condition where the cornea or lens is curved irregularly, leading to light focusing unevenly on the retina.
The CYL value indicates the additional lens power needed to correct this uneven curvature, while the Axis specifies the orientation or angle of that correction. Unlike SPH, which applies uniformly, CYL power is only added along a specific meridian of the lens, defined by the Axis number. If a person has no astigmatism, the CYL and Axis columns will be left blank or marked with a dash or “SPH.”
The presence of a non-zero CYL value alongside the SPH means the lens must have a toroidal or spherocylindrical shape, combining the uniform spherical power with the localized cylindrical power. SPH remains the foundational measurement for nearsightedness or farsightedness, with CYL and Axis refining the focus for eyes with asymmetrical shapes.