The term “spherical” (SPH) on an eyeglass prescription refers to the fundamental lens power required to correct general focusing errors. It indicates the basic degree of nearsightedness or farsightedness that needs to be addressed for clear vision. Understanding this number is the first step in deciphering the instructions used to craft your corrective lenses.
Defining Spherical Correction
A spherical lens has a uniform curvature across its entire surface, much like a slice taken from a ball. This consistent curvature means the lens bends light equally in all directions, providing the basic correction for the eye’s refractive error.
The power of this correction is measured in units called diopters (D). The higher the absolute number of the spherical power, the stronger the correction required to bring light into focus. This measurement indicates the lens’s ability to converge or diverge light to ensure it lands precisely on the retina.
Connecting Spherical Power to Common Vision Issues
The spherical power addresses two common refractive errors: myopia and hyperopia. The prescription number is preceded by a minus sign (–) or a plus sign (+) to indicate which condition is being corrected. These signs determine how the lens must shift the eye’s natural focal point.
A negative (–) SPH value indicates myopia, or nearsightedness. In a myopic eye, light focuses too far in front of the retina, causing distant objects to appear blurry. The concave (minus) spherical lens diverges light rays, pulling the focal point backward onto the retina.
A positive (+) SPH value corrects for hyperopia, or farsightedness. This occurs when light focuses theoretically behind the retina. The convex (plus) spherical lens converges light rays, pushing the focal point forward to land correctly on the retina. This correction helps farsighted individuals see nearby objects more clearly.
Spherical Correction vs. Cylindrical Correction
Spherical power is distinct from cylindrical power (CYL), another common component of an eyeglass prescription. While the spherical lens has a uniform curve, the cylindrical lens has a non-uniform, or toric, curvature, meaning it is curved along only a single axis. This difference reflects the vision problem each corrects.
Spherical correction addresses overall focal errors, causing a consistent blur. Cylindrical correction, however, treats astigmatism, a condition where the cornea or lens is shaped unevenly. Astigmatism causes light to focus unevenly, requiring a lens that corrects power along a specific orientation.
If a prescription only contains an SPH value, the eye requires only uniform, spherical correction. If both SPH and CYL values are present, the eye has both a general focusing error and an uneven curvature. The cylindrical power works with the spherical power to fine-tune the focus where the unevenness exists.