Reading an eyeglass prescription often feels like deciphering a cryptic code, filled with unfamiliar abbreviations and numbers. The term “power” is frequently used to describe the strength of a lens, but on a prescription, this concept is broken down into specific measurements. Understanding the relationship between the general term “power” and the abbreviation SPH, or Sphere, is a common source of confusion. SPH represents a particular type of lens correction, but it is not always the only measurement determining the overall vision correction provided by the lens.
Understanding Optical Power
Optical power is the measurement of a lens’s ability to bend light, a necessary function to ensure light focuses correctly onto the retina at the back of the eye. This refractive capability is universally quantified using a unit called the Diopter, abbreviated as D. A lens with a higher Diopter value bends light more sharply, indicating a stronger requirement for vision correction. The number in the prescription, regardless of the sign preceding it, represents the strength of the required lens. The use of the Diopter provides a standardized, scientific method for eye care professionals to communicate the precise degree of correction needed to achieve clear vision.
What Does SPH Specifically Measure
SPH is the abbreviation for “Sphere,” which refers to the spherical component of the lens power. This value indicates the lens strength required to correct myopia (nearsightedness) or hyperopia (farsightedness). The term “spherical” signifies that the correctional power is uniform across all meridians of the lens. If the number in the SPH column is preceded by a minus sign (-), the lens corrects for myopia, where light focuses in front of the retina. Conversely, a plus sign (+) denotes correction for hyperopia.
SPH vs Total Corrective Power
The answer to whether SPH is the same as the total power is that it can be, but often it is not. The SPH value represents the total corrective power only in cases where the eye is perfectly spherical, meaning the prescription includes no correction for astigmatism. Astigmatism occurs when the cornea or lens is irregularly shaped, causing light to focus unevenly. When astigmatism is present, the total corrective power of the lens becomes a combination of the SPH value and the CYL, or Cylinder, value. The Cylinder value specifies the additional lens power required to correct the asymmetry caused by astigmatism.
Other Factors in Vision Correction
The SPH value is just one of several factors that contribute to a complete vision correction strategy. Beyond the SPH, a full prescription may include the CYL and AXIS measurements, which specifically address astigmatism. The CYL value quantifies the degree of astigmatism correction needed, while the AXIS is a number between 1 and 180 that defines the precise orientation on the lens where the cylindrical correction must be applied. Additionally, people over the age of 40 may see an ADD, or Addition, power value on their prescription. This is the extra Diopter strength required to compensate for presbyopia, the age-related loss of near focusing ability.