A patient often assumes their glasses prescription numbers will transfer directly to contact lenses. While the goal is the same—to achieve clear vision—the optical principles governing glasses and contacts are distinct, meaning the numbers on the two prescriptions are frequently not identical. The power required for contact lenses is related to the spectacle prescription, but a final, usable contact lens prescription must be determined and finalized by an eye care professional (ECP). This is due to necessary power adjustments and the inclusion of specific parameters for lens fit and safety. This conversion process accounts for the differing positions of the lenses relative to the eye.
Understanding Prescription Terminology
A standard glasses prescription contains several measurements quantifying the necessary vision correction, all measured in diopters (D). The Sphere (SPH) value indicates the main refractive correction for nearsightedness or farsightedness. A minus sign (-) signifies myopia (nearsightedness), where distant objects appear blurry. Conversely, a plus sign (+) denotes hyperopia (farsightedness), where close-up objects are often blurry.
The Cylinder (CYL) and Axis values are included only for patients who have astigmatism, a condition where the cornea is shaped more like a football than a sphere. The CYL number specifies the additional lens power needed to correct this irregular curve. The Axis is a number between 1 and 180 degrees that tells the lens manufacturer the precise orientation of that cylindrical power on the lens. Finally, the Add value, or Addition, represents the extra magnifying power needed for reading or near-vision tasks, typically used for bifocal or multifocal lenses.
Why Glasses and Contact Prescriptions Differ
The primary reason a glasses prescription cannot be used directly for contact lenses is a concept known as Vertex Distance. This is the physical gap between the back surface of the glasses lens and the front of the eye’s cornea. Glasses typically sit about 12 to 14 millimeters away from the eye, which is the distance at which the prescription was measured.
Contact lenses, however, rest directly on the surface of the cornea, effectively making the vertex distance zero. Changing the distance of a lens from the eye alters its effective power, meaning the light path is slightly different when using contacts compared to glasses. This difference is negligible for low-power prescriptions, but for stronger prescriptions, the change in lens position requires a mathematical adjustment to maintain the same visual clarity.
Calculating the Sphere Power Conversion
The adjustment for vertex distance is primarily applied to the Sphere power, and this conversion is generally only necessary for prescriptions stronger than plus or minus 4.00 Diopters (D). For powers weaker than this threshold, the optical difference caused by the change in distance is typically too small to warrant a power change in the commercially available contact lens powers. The calculation involves a formula relating the spectacle power, the vertex distance in meters, and the resulting contact lens power.
When converting a negative (myopic) glasses prescription to a contact lens prescription, the power of the contact lens will become numerically weaker. For instance, a patient with a glasses prescription of -6.00 D might require a contact lens power closer to -5.50 D since the lens is closer to the eye. Conversely, a positive (hyperopic) glasses prescription requires a numerically stronger contact lens power when moved from the glasses frame to the cornea. A patient wearing +8.00 D glasses might need a contact lens power of approximately +8.75 D to achieve the same visual result.
The Cylinder power, which corrects astigmatism, is not converted in the same way as the Sphere power. The combination of sphere and cylinder power creates two different power meridians, requiring a complex calculation known as vertex compensation for sphero-cylindrical prescriptions. Most soft contact lenses are only available in a limited range of cylinder powers and axes, often requiring the eye care professional to select the closest available option. Therefore, the accurate conversion and selection of astigmatism correction remains the responsibility of the eye care professional.
Contact Lens Specific Parameters
The glasses prescription only contains the refractive power needed for correction, but a contact lens prescription must include several additional parameters that relate to the physical fit on the eye. The Base Curve (BC) is a measurement, typically in millimeters, that dictates the curvature of the back surface of the contact lens. This value is chosen to match the curvature of the patient’s cornea to ensure the lens rests comfortably and moves correctly on the eye.
The Diameter (DIA) measures the total width of the lens, ensuring it is the appropriate size to cover the cornea and surrounding area. A lens that is too small or too large can cause discomfort, blurry vision, or compromise the health of the eye. The contact lens prescription also specifies the brand, material, and Modality (such as daily or monthly disposable), factors determined during a comprehensive fitting. Because these parameters relate to the individual shape and health of the eye, a formal fitting and examination are required to finalize a contact lens prescription, ensuring safety and optimal vision.