An eye prescription is a standardized medical document provided by an eye care professional after a comprehensive vision examination. This prescription details the exact optical power required to correct a person’s refractive errors. The form uses universal abbreviations and numerical values to communicate complex measurements. Understanding these terms, particularly the Sphere (SPH) and Cylinder (CYL) values, is the first step toward knowing what kind of vision correction is needed.
Defining Sphere (SPH)
SPH represents the main corrective power needed to fix nearsightedness or farsightedness. This value indicates a uniform, or spherical, degree of correction that is applied evenly across the entire surface of the lens. The measurement is given in diopters (D), the unit of optical power required to focus light onto the retina.
The sign preceding the SPH number is a simple indicator of the primary refractive error. A negative number (–) indicates myopia, or nearsightedness, meaning distant objects appear blurred. Conversely, a positive number (+) signifies hyperopia, or farsightedness, where close-up objects are difficult to focus on. The further the number is from zero, regardless of the sign, the stronger the lens power required to achieve clear vision.
Defining Cylinder (CYL)
The Cylinder, or CYL, value is the specific amount of lens power designated to correct astigmatism. This condition occurs when the cornea or lens has an irregular, non-spherical shape. This irregular curvature causes light to focus at multiple points inside the eye, resulting in blurred or distorted vision at various distances.
The CYL value, also measured in diopters, specifies the additional power needed along a particular meridian of the lens to compensate for this unevenness. If the CYL value is blank or reads “0.00,” the eye has no measurable astigmatism or the degree is too slight to require correction. The power listed under CYL is always an additive correction to the base SPH power, addressing the difference in curvature between the eye’s steepest and flattest meridians.
The Context of Other Prescription Values
The CYL value is never listed in isolation because its corrective power must be placed at a precise angle on the lens. This requires the Axis (AX) value, which indicates the orientation of the astigmatism correction in degrees, ranging from 1 to 180. The Axis number tells the lens maker exactly where to align the cylindrical power to counteract the irregular curve of the eye.
Eye Identification (OD and OS)
The entire set of values—SPH, CYL, and Axis—are assigned to specific eyes, identified by the Latin abbreviations OD and OS. OD (oculus dexter) refers to the right eye, and OS (oculus sinister) refers to the left eye.
Addition (ADD)
Another value is ADD, which stands for Addition. This represents the extra magnifying power needed for reading or close-up tasks. This value is typically included for patients requiring bifocal or progressive lenses.