It is a common misunderstanding that a prescription for eyeglasses is interchangeable with one for contact lenses. While both correct vision, they are fundamentally different because of how the devices interact with the eye. An eyeglass prescription alone is not sufficient to order contact lenses, as contacts are medical devices that sit directly on the eye’s surface. Obtaining contact lenses requires a specialized fitting and evaluation process to ensure a proper, healthy, and comfortable fit based on the eye’s unique shape and health.
The Necessity of a Separate Contact Lens Exam
The primary reason a glasses prescription cannot be used for contacts relates to vertex distance. Eyeglass lenses are positioned approximately 12 to 14 millimeters away from the cornea. This distance causes the effective optical power of the lens to differ from the power needed when the lens sits directly on the eye, where the vertex distance is zero.
For people with stronger refractive errors, typically exceeding plus or minus 4.00 diopters, the eye care professional must perform vertex compensation. This adjustment is important because a contact lens that is too strong or too weak will not provide clear vision. For example, a high minus prescription for nearsightedness usually requires a slightly lower power in a contact lens compared to the glasses lens.
Contact lenses are medical devices that make direct contact with the corneal tissue. The fit must be precise to allow for proper oxygen flow and tear exchange beneath the lens, which is necessary for maintaining eye health. An improperly fitted contact lens can cause discomfort, irritation, and potentially serious complications like corneal abrasions or infections, making the physical fit a matter of health, not just vision correction.
The Contact Lens Fitting and Evaluation Process
To determine the correct fit, the eye care professional begins by measuring the curvature of the cornea using an instrument called a keratometer. This measurement is crucial because the contact lens must conform to the eye’s surface, and the data gathered helps determine the lens’s necessary base curve (BC). In some cases, a corneal topographer may be used to create a detailed map of the entire corneal surface, which is particularly useful for fitting patients with astigmatism or irregular corneal shapes.
The provider also checks the overall health of the eye, looking for conditions like dry eye or allergies that might affect the ability to wear lenses comfortably. The quality and quantity of the tear film are assessed, as adequate tears are necessary to keep the lenses hydrated and prevent dryness or irritation. Lifestyle factors, such as the patient’s daily activities and preferences for daily disposable or extended-wear lenses, are also discussed during this initial evaluation.
Following the measurements, the patient is given trial lenses to wear for a short period. The eye care professional then assesses the fit by observing how the lens sits on the eye, checking for proper centration, and evaluating its movement with each blink. A lens that moves too much is too flat, while one that moves too little is too steep, and both can limit tear exchange and cause discomfort.
The trial process allows the patient to confirm visual acuity and comfort before the final prescription is issued. Once a satisfactory lens is found, the patient is instructed on the proper techniques for insertion, removal, and basic cleaning and care. This process ensures the selected lens is safe, comfortable, and provides optimal vision correction.
Understanding the Final Contact Lens Prescription
A contact lens prescription contains several unique parameters not found on an eyeglasses prescription, reflecting the need for a precise physical fit on the eye. The Base Curve (BC) specifies the curvature of the back surface of the lens, which is determined by the corneal measurements taken during the fitting. This is often measured in millimeters and ensures the lens rests correctly on the cornea.
Another specific measurement is the Diameter (DIA), which indicates the width of the lens and is also measured in millimeters. The diameter is chosen to ensure the lens adequately covers the cornea while remaining comfortable. Both the Base Curve and Diameter are essential for the physical health of the eye and the stability of the lens.
Crucially, a contact lens prescription must also specify the Brand or Manufacturer of the lens. Unlike glasses lenses, contact lenses are brand-specific because different manufacturers use proprietary materials and designs that affect the fit, oxygen permeability, and overall comfort. Even if two brands have the same power, their physical fit might be completely different.
Safe and Legal Purchasing Options
In the United States, the Fairness to Contact Lens Consumers Act (FCLCA) governs how prescriptions are handled and purchased. This federal law ensures that consumers have the right to shop around and buy their lenses from any seller, including licensed optometrists, ophthalmologists, or reputable online retailers. The prescription issued after the fitting is typically valid for one year, though the duration can vary based on state law or the eye care professional’s medical judgment.
When purchasing lenses, sellers are required to verify the prescription with the eye care provider before dispensing the contacts. This verification process is mandated to ensure that the lenses being purchased match the specific brand, power, base curve, and diameter prescribed for the patient’s eyes. If the prescriber does not respond to the seller’s verification request within eight business hours, the prescription is automatically verified, and the seller can ship the lenses.
It is advised to avoid purchasing lenses from unverified sources, particularly decorative or cosmetic lenses sold without a prescription. These lenses are considered medical devices by the Food and Drug Administration (FDA) and can pose significant health risks if not properly fitted. Purchasing from established, legal channels and adhering to the specific parameters on the valid prescription helps safeguard vision and eye health.