Many consumers wonder if they can use their existing eyeglass prescription to purchase contact lenses. A prescription for glasses is not legally or medically sufficient to obtain them. While both correct the same refractive error, contact lenses are classified as medical devices that sit directly on the eye’s delicate surface. This requires a distinct evaluation process to ensure proper physical fit, accurate power correction, and eye health. A professional fitting with an eye care specialist is mandatory before a contact lens prescription can be issued.
Why Glasses Prescriptions Are Insufficient
The difference between the two types of vision correction lies in the distance between the lens and the eye, known as the vertex distance. Eyeglass lenses typically sit about 12 to 14 millimeters away from the cornea, while a contact lens rests directly on the eye’s surface, resulting in a zero vertex distance. This difference in placement changes the optical power required for clear vision because light bends over a different distance before reaching the eye.
For individuals with milder prescriptions, the difference in power may be negligible. However, for stronger prescriptions, generally exceeding plus or minus 4.00 diopters, the power must be mathematically adjusted. This calculation, called vertex compensation, ensures the correct corrective strength is ordered for the contact lens. For example, a glasses power of -6.00 diopters would translate to a weaker power for a contact lens to provide the same visual acuity.
Glasses prescriptions only include the sphere, cylinder, and axis values, which relate purely to correcting the refractive error. They contain no information about the physical dimensions of the eye, which is necessary for a safe fit. Using an unadjusted power or an improperly fitted lens can lead to blurred vision, discomfort, or complications. Because contact lenses physically interact with the cornea, a separate fitting is necessary to protect eye health.
Measurements Unique to Contact Lenses
A contact lens prescription includes specialized measurements absent from a glasses prescription. The two most prominent are the Base Curve (BC) and the Diameter (DIA). The Base Curve measures the curvature of the back surface of the lens. This measurement must be precisely matched to the curvature of the wearer’s cornea for a proper fit.
The Base Curve measurement, given in millimeters, typically falls between 8.3 and 9.0 mm for soft lenses and determines how the lens rests on the eye. A lens with an improper Base Curve can be too tight, restricting oxygen flow and causing discomfort. Conversely, a lens that is too loose leads to excessive movement and blurry vision. The Diameter specifies the overall width of the lens, ensuring it covers the proper area of the cornea and sclera.
Soft contact lens diameters usually range from 13.0 to 14.5 mm, with 14.0 mm being common. Beyond physical dimensions, a contact lens prescription must specify the exact brand, manufacturer, and material of the lens. Different lens materials have varying levels of oxygen permeability and water content. This affects how the eye responds to the lens, making the specific product a regulated part of the prescription.
Legal Requirements for Contact Lens Prescriptions
Contact lenses are regulated products, and federal rules mandate that a seller may only dispense lenses based on a valid prescription. The law requires that a prescriber must give the patient a copy of their contact lens prescription immediately upon completion of the fitting. This right, established by the Fairness to Contact Lens Consumers Act, allows patients to shop around and purchase lenses from any retailer.
A contact lens prescription must have a minimum expiration date of one year from the date of the fitting. Prescribers may set a shorter expiration date only if there is a documented medical reason related to the patient’s ocular health. When a patient orders lenses from a third-party retailer, the retailer must initiate a verification process with the prescriber.
The retailer submits the patient and lens information, and the prescriber has an allotted time, typically eight business hours, to respond and confirm the prescription’s accuracy. If the prescriber fails to respond within that timeframe, the prescription is considered automatically verified, and the seller can proceed with the order. This verification system ensures consumers receive the correct medical device while allowing freedom of choice in purchasing.