As people age, the ability of the eye’s natural lens to focus on close objects gradually declines, a common condition known as presbyopia. This change typically begins around age 40, making small print and close-up tasks increasingly difficult. To restore comfortable near vision, many people turn to reading glasses, often seeking inexpensive over-the-counter (OTC) options. Understanding how to translate a comprehensive distance prescription into a usable reading power is the first step toward finding the correct magnification strength.
Deciphering Key Prescription Terms
A standard eyeglass prescription contains several measurements, but only two are needed to determine the reading power: Sphere (SPH) and Addition (ADD). The SPH value, measured in diopters (D), represents the main distance correction needed for either nearsightedness or farsightedness. A minus sign (-) before the SPH number indicates nearsightedness, while a plus sign (+) or no sign indicates farsightedness.
The ADD, or Addition, is always a positive value representing the extra focusing power required for near work. This value is the boost necessary to overcome presbyopia and is typically the same for both the right and left eyes. The ADD power is usually listed once on the prescription, separate from the SPH, and commonly ranges from +0.75 D to +3.00 D.
Diopters are the unit of measure for the corrective strength of the lens, with increments typically increasing by 0.25 D. This standardized measurement allows for precise lens manufacturing to correct the refractive error defined by the SPH and ADD values.
The Fundamental Calculation for Reading Power
The total power required for custom reading glasses is found by combining the eye’s existing distance correction with the extra focusing power needed for close work. This relationship is expressed by the formula: Distance Correction (SPH) + Near Addition (ADD) = Total Reading Power. This calculation must be performed separately for each eye to account for any difference in the SPH value.
For example, a person with no distance correction (SPH of 0.00) and an ADD of +2.00 would have a total reading power of +2.00 D. If a person is farsighted with an SPH of +1.00 and an ADD of +1.50, the reading power is calculated as +1.00 + +1.50 = +2.50 D. In both cases, the final reading strength is a positive diopter value.
The calculation changes when the SPH is negative, indicating nearsightedness. For a person with an SPH of -1.00 and an ADD of +2.50, the calculation is -1.00 + +2.50 = +1.50 D, resulting in a lower total reading power. Conversely, a person with an SPH of -2.00 and an ADD of +1.50 would have a reading power of -0.50 D. This negative result means they likely need no reading glasses and could simply remove their distance glasses for near tasks.
The result of this calculation represents the precise diopter strength for a custom reading lens. It is important to perform this addition for both the right eye (OD) and the left eye (OS), as the SPH values may differ even though the ADD value is typically the same.
Limitations of Over-the-Counter Readers
The precise power calculated from your prescription is intended for custom lenses, and applying this number to generic store-bought readers introduces several limitations. Over-the-counter (OTC) reading glasses are a mass-produced, one-size-fits-all solution, meaning they have the exact same power in both the right and left lenses. This uniformity cannot accommodate the common scenario where a person has different SPH values in each eye, potentially leading to eye strain.
A significant limitation is that OTC readers do not correct for astigmatism, which is measured by the Cylinder (CYL) and Axis values on a prescription. Astigmatism is an imperfection in the eye’s curvature. Without the precise cylindrical correction, vision will not be as sharp as with a custom lens, even at the correct power, as these generic magnifiers are designed only to address the spherical error caused by presbyopia.
The calculated reading power assumes a standard close working distance, typically around 14 to 16 inches. An individual’s actual working distance—how far they hold a book or smartphone—can affect the required power. If the calculated power falls between the standard increments of OTC readers (e.g., +1.75 D), it is recommended to choose the lower available strength, like +1.50 D, to avoid over-magnification and eye strain.