Bifocal prescriptions correct vision at two distinct distances, typically for far and near viewing. Understanding the components of your prescription empowers you to comprehend your vision needs. A bifocal lens combines two prescriptions within a single lens, with the top section for distance and the lower section for near vision.
Decoding Standard Prescription Terms
An eyeglasses prescription contains abbreviations and numerical values that specify vision correction. These terms serve as the foundation for any prescription, including bifocals. Each eye is listed separately: “OD” for the right eye (oculus dexter) and “OS” for the left eye (oculus sinister). “OU” (oculus uterque) may appear if the prescription is identical for both eyes.
The “SPH” or “Sphere” value indicates the main lens power needed to correct nearsightedness or farsightedness. This measurement is in diopters, where a minus sign (-) signifies nearsightedness (difficulty seeing distant objects) and a plus sign (+) denotes farsightedness (difficulty seeing close objects). The further the number is from zero, the stronger the lens correction required.
“CYL” or “Cylinder” addresses astigmatism, a condition where the eye’s cornea has an irregular, more oval shape. If no astigmatism is present, this field may be left blank or contain “PL” for plano. The “AXIS” number, ranging from 1 to 180 degrees, accompanies the CYL value and specifies the orientation of the astigmatism correction on the lens. Without a CYL value, there will be no AXIS.
Some prescriptions may also include “PRISM” and “BASE” values, which correct eye alignment issues or double vision. Prism indicates the amount of prismatic power, while Base specifies the direction, such as base up (BU), base down (BD), base in (BI), or base out (BO). These are less common but important for specific visual challenges.
Understanding the “ADD” Power
The “ADD” power, sometimes labeled “NV-ADD,” is a specific component of a bifocal prescription. This value represents the additional magnifying power required for near vision tasks like reading or using a smartphone. The need for this additional power arises from presbyopia, an age-related condition where the eye’s natural lens gradually loses its ability to focus on close objects.
The ADD value is always a positive number, typically ranging from +0.75 to +3.50 diopters. This value is usually the same for both eyes, even if the distance prescriptions differ. The body’s accommodative response, or ability to focus at near distances, is generally symmetrical between the eyes, making a single ADD value sufficient.
It ensures clear vision at close range by supplementing the distance prescription. An eye care professional determines this value during an eye examination based on the individual’s near vision needs.
Interpreting Your Complete Bifocal Prescription
Reading a complete bifocal prescription involves synthesizing the distance correction with the additional near vision power. The prescription typically presents the distance values (SPH, CYL, AXIS) for each eye (OD and OS). The “ADD” power is usually listed in a separate column or section, often appearing only once since it applies to both eyes.
To determine your total near vision power for each eye, you add the “ADD” value to the “SPH” value of your distance prescription. For example, if your OD distance SPH is -2.00 and your ADD is +2.50, your near vision SPH for the right eye would be +0.50. Similarly, if your OS distance SPH is +1.00 and your ADD is +2.00, your near vision SPH for the left eye would be +3.00. The CYL and AXIS values typically remain the same for both distance and near portions of a bifocal lens.
Prescription layouts can vary, with some listing the ADD power under “NV-ADD” or simply “Add.” Sometimes, the near power is explicitly calculated and listed separately for each eye, but the underlying ADD value remains consistent.
Understanding your prescription provides insight into your vision correction. However, for specific questions, concerns, or interpretation of complex prescriptions, consulting a qualified optometrist or ophthalmologist remains the best approach. They can provide personalized guidance and ensure your eyeglasses meet your precise visual requirements.