Switching from glasses to contact lenses is not a simple transfer of prescription numbers. Both corrective devices focus light onto the retina, but the physical proximity of the lens to the eye fundamentally changes the required optical power. Converting a glasses prescription (Rx) to a contact lens Rx requires specific measurements, physical fitting requirements, and mathematical adjustments. This calculated modification is necessary for optimal eye health and visual acuity.
Why Glasses and Contact Lens Prescriptions Differ
The primary reason a glasses prescription cannot be used directly for contacts is vertex distance, the space between the corrective lens and the cornea. Spectacle lenses are typically positioned 12 to 14 millimeters away from the eye’s surface, which impacts how the eye perceives the lens power. Contact lenses sit directly on the tear film layer, meaning their vertex distance is essentially zero. To achieve the same focal point as glasses, the contact lens power must be adjusted.
The closer a lens is to the eye, the less power it needs to correct high nearsightedness (myopia), and the more power it needs for high farsightedness (hyperopia). This shift in required power becomes significant as the magnitude of the prescription increases. For lower prescriptions, the difference is negligible, but for higher refractive errors, the mathematical change is substantial.
Unique Measurements Found Only on a Contact Lens Prescription
Contact lenses are medical devices that interact directly with the cornea, requiring additional physical measurements absent from a glasses prescription. These parameters are necessary for a proper and healthy fit.
The Base Curve (BC) defines the curvature of the lens’s back surface. This measurement must closely match the curvature of the cornea to ensure the lens rests securely. If the fit is too tight, it can restrict oxygen flow; if too loose, it can cause excessive movement and discomfort. BC is measured in millimeters (mm).
The Diameter (DIA) specifies the overall width of the lens, also measured in millimeters. The diameter ensures the lens properly covers the cornea and positions the optical center correctly over the pupil, ensuring stability and centration. Furthermore, the specific Material/Brand is listed on the prescription, as different materials, like hydrogel or silicone hydrogel, affect oxygen permeability and the required wearing schedule.
Adjusting the Power for Vertex Distance
The mathematical adjustment of lens power is the core step in converting a glasses prescription. This conversion is typically required only when the spherical power (SPH) is approximately \(\pm 4.00\) Diopters or greater.
When converting a high minus prescription (nearsightedness), the contact lens power must be reduced (made less negative) because the lens is closer to the eye. Conversely, for a high plus prescription (farsightedness), the contact lens power must be increased (made more positive) to maintain the correct focus.
Practitioners use specialized vertex distance conversion charts or formulas to calculate the new spherical power accurately. If the prescription includes astigmatism, the cylindrical power (CYL) and axis may need modification. Alternatively, the cylinder power may be converted to a spherical equivalent if a spherical contact lens is used.
Why You Need a Specific Contact Lens Fitting
Even after the mathematical power conversion, a contact lens prescription is legally and medically separate from a glasses prescription and requires a specific fitting process. The eye care professional selects trial lenses based on the calculated power and initial corneal measurements.
During the fitting, the practitioner evaluates the physical interaction between the lens and the eye. They verify that the Base Curve and Diameter provide an appropriate fit, assessing lens movement and centration. They also check the integrity of the cornea for potential complications like swelling or oxygen deprivation.
A contact lens prescription is not complete until this physical assessment is conducted and verified with a trial period. This mandatory evaluation ensures the lens is safe, comfortable, and provides stable vision. The final prescription specifies the exact power, BC, DIA, material, and expiration date.