An eye prescription is a precise map of the visual correction needed to focus light accurately onto the retina. The numbers and abbreviations are standardized measurements defining the power, shape, and orientation of the lenses required for clear vision. Understanding how to interpret and convert these values is useful for anyone purchasing eyewear, whether for glasses that sit a distance from the eye or for contact lenses that rest directly on the cornea.
Deciphering the Standard Prescription Terminology
The first step in understanding an eye prescription is identifying which eye is being addressed using Latin abbreviations. OD (Oculus Dexter) refers to the right eye, and OS (Oculus Sinister) refers to the left eye. These distinctions are necessary because each eye often requires a different amount of correction. All power values are measured in diopters (D).
The Sphere (SPH) value is the primary correction for nearsightedness or farsightedness, applying the same power across the entire lens. A minus sign (-) signifies nearsightedness (myopia), where distant objects are blurry. A plus sign (+) indicates farsightedness (hyperopia), where close-up vision is more difficult.
Correction for astigmatism is provided by the Cylinder (CYL) and Axis values, which address an irregularly shaped cornea. The CYL number specifies the lens power needed to correct this irregularity. If this column is blank, the eye requires no astigmatism correction.
The Axis is a number between 1 and 180 degrees that tells the lab the precise orientation of the CYL correction on the lens. It acts like a protractor, with 90 degrees representing the vertical meridian and 180 degrees representing the horizontal meridian. A few prescriptions may also include a Prism value, measured in prism diopters, which is a specialized correction for eye alignment issues that cause double vision.
Converting Between Plus and Minus Cylinder Notation
Eyeglass prescriptions for astigmatism can be written in two optically equivalent formats: minus cylinder or plus cylinder notation. Most eye doctors use minus cylinder notation, but converting between the two forms, a process called transposition, maintains the exact optical power of the correction. Transposition involves a three-step algebraic process.
The first step is to calculate the new Sphere power by algebraically adding the original Sphere and Cylinder powers together. For example, starting with a prescription of +2.50 -2.00 x 105, the new Sphere power is +2.50 + (-2.00), which equals +0.50.
The second step requires changing the sign of the original Cylinder power while keeping its absolute value the same. The original -2.00 CYL becomes +2.00 CYL in the transposed form.
The final step is to adjust the Axis by exactly 90 degrees. If the original Axis is 90 degrees or less, add 90 degrees; if it is greater than 90 degrees, subtract 90 degrees. In the example, 105 is greater than 90, so subtracting 90 results in a new Axis of 15. Therefore, the transposed prescription becomes +0.50 +2.00 x 15, providing the identical optical correction.
Adjusting Power for Contact Lenses Versus Glasses
A glasses prescription and a contact lens prescription are not interchangeable because of the difference in the lens’s distance from the eye. Glasses sit about 12 to 14 millimeters away (vertex distance), while contact lenses rest directly on the cornea.
This difference changes the effective power of the lens, requiring a calculation called vertex compensation for higher-strength prescriptions. For Sphere powers stronger than ±4.00 diopters, the adjustment is significant enough to alter the required lens strength.
For nearsightedness (minus powers), the contact lens prescription will be slightly weaker than the glasses prescription (e.g., -6.00 glasses converts to -5.75 contacts). Conversely, for farsightedness (plus powers), the contact lens power will need to be slightly stronger than the corresponding glasses power. While the mathematical formula exists, an eye care professional must perform this precise conversion as part of a contact lens fitting, which also determines the necessary base curve and diameter.
Calculating Near Vision Power (The ADD Value)
The ADD value, short for “Addition,” indicates the extra magnifying power needed for focusing on close objects. This requirement is associated with presbyopia, an age-related change that causes the eye’s natural lens to lose flexibility, making near tasks difficult for those typically over age 40. The ADD power is always a positive number and is usually the same for both eyes.
This additional power is incorporated into multifocal lenses, such as bifocals and progressives, or used for single-vision reading glasses. To determine the total reading power, the ADD value is added to the Sphere component of the distance prescription. For instance, if the distance SPH is -1.00 and the ADD is +2.00, the resulting SPH power for near vision is +1.00.
The CYL and AXIS values, which correct astigmatism, remain unchanged when calculating the total near vision power. The ADD power typically ranges from +1.00 to +3.50 diopters, increasing as the eye’s focusing ability declines with age.