Eyeglass prescriptions often appear confusing due to the standardized medical shorthand used by optometrists and ophthalmologists. This form is a precise communication document designed to instruct a lens laboratory on how to grind specific corrective power into lenses. Bifocal prescriptions are unique because they contain distinct measurements to correct vision at two different viewing distances. Understanding this document requires breaking down the structured grid into its individual components.
The Standard Prescription Grid (Distance Correction)
The top section of any bifocal prescription forms the basis for distance vision correction, often referred to as the “Distance” or “D.V.” section. This part of the grid includes the Sphere (SPH) measurement, which indicates the main strength of the lens power required. A minus (-) sign in front of the SPH value corrects for myopia, or nearsightedness, while a plus (+) sign addresses hyperopia, or farsightedness. This spherical power is the fundamental adjustment needed to focus light directly onto the retina for clear distance viewing.
Alongside the Sphere is the Cylinder (CYL) value, which is present only when a patient has astigmatism. Astigmatism occurs when the cornea or lens is irregularly curved, causing light to focus unevenly and resulting in distorted vision at all distances. The CYL value specifies the exact amount of additional lens power necessary to compensate for this uneven curvature.
The final component of distance correction is the Axis, which is always listed between 1 and 180 degrees. This number designates the precise orientation or angle at which the cylindrical power must be placed within the lens. The Axis dictates where the astigmatism correction needs to be applied to neutralize the specific curvature of the eye.
Identifying the Bifocal Power (The ADD Column)
The definitive feature of a bifocal prescription is the ADD power, which stands for Near Addition or Added Power. This single measurement directly addresses presbyopia, the age-related loss of the eye’s ability to focus on close-up objects. The ADD value represents the supplementary magnifying power needed specifically for reading, computer work, or other tasks requiring near vision. This addition power is consistently a positive (+) value because it involves introducing convexity to the lens to increase magnification and shorten the focal point.
The power typically ranges from +0.75 to +3.00 Diopters, depending on the severity of the presbyopia and the patient’s age. This measurement is usually identical for both the right and the left eye, simplifying the bifocal design process.
To determine the total power for the reading segment of the bifocal lens, the ADD power is mathematically combined with the Sphere (SPH) value from the distance correction section. For example, a distance SPH of -2.00 combined with an ADD power of +2.00 results in a net power of 0.00 for reading. This additive calculation is performed by the lens lab to create the distinct power difference between the top and bottom segments of the bifocal lens.
Decoding the Optical Language (Key Abbreviations)
The prescription form relies heavily on traditional Latin abbreviations to denote which eye is being measured. OD stands for Oculus Dexter, referencing the patient’s right eye, and is always listed first on the document. Following this is OS, which means Oculus Sinister, specifying the measurements for the patient’s left eye. Sometimes the letters OU, standing for Oculus Uterque, may be used to indicate a measurement that applies uniformly to both eyes.
Understanding the symbols is important for interpreting the power values. The plus sign (+) indicates a convex lens used to correct farsightedness (hyperopia), while the minus sign (-) denotes a concave lens needed for nearsightedness (myopia). All corrective powers on the prescription are measured in units called Diopters (D), which is the standardized metric for lens power and focal length in optometry.