An eyeglass prescription cannot be used directly to purchase contact lenses because the two vision correction methods rely on different sets of measurements. While your eyeglass prescription accurately measures your refractive error, a contact lens prescription requires additional parameters related to the physical fit of the lens on the eye. The difference lies in the distance between the corrective lens and the cornea, which significantly affects the required optical power. Consequently, a separate fitting and evaluation process is mandatory to ensure safe and effective contact lens wear.
Power Adjustments and Vertex Distance
The primary reason for the difference in lens power between glasses and contacts is vertex distance. This distance is the space between the back surface of an eyeglass lens and the front surface of the eye’s cornea, typically around 12 to 14 millimeters. Because an eyeglass lens sits a short distance away from the eye, its effective optical power is slightly different than its measured power.
A contact lens sits directly on the cornea, which means it has a vertex distance of virtually zero. This change in position requires a recalculation of the lens’s power to maintain the same visual correction. For lower prescriptions, generally those under plus or minus 4.00 diopters (D), this difference is often negligible, and the power number remains the same.
However, for higher prescriptions, the change in position is significant and necessitates a vertex distance conversion. For example, a spectacle lens with a power of -8.00 D may need to be converted to a contact lens power of approximately -7.50 D to correct vision accurately at the corneal plane. This adjustment ensures the light is focused precisely on the retina, preventing blurriness, eyestrain, and headaches.
Physical Parameters Unique to Contacts
Beyond the optical power, a contact lens prescription must include physical measurements that have no equivalent on a glasses prescription. These measurements ensure the lens conforms correctly to the unique shape of the eye for comfort and health. The two most prominent of these are the Base Curve (BC) and the Diameter (DIA).
The Base Curve, measured in millimeters, determines the curvature of the back surface of the contact lens, which must match the curvature of the cornea. Soft contact lenses typically have a BC between 8.3 and 9.0 millimeters, with a higher number indicating a flatter curve. An improperly steep (low BC) or flat (high BC) lens can lead to discomfort, poor vision, or even corneal damage.
The Diameter is the total width of the lens from edge to edge and ensures the lens adequately covers the cornea. For soft lenses, the Diameter usually falls between 13.8 and 14.5 millimeters. If the Diameter is too small, the lens may not cover the cornea fully, leading to an unstable fit, while a lens that is too large can interfere with the eyelids and cause irritation. Selecting the correct Base Curve and Diameter is important for allowing sufficient oxygen to reach the cornea, which maintains long-term eye health.
The Necessary Contact Lens Fitting and Exam
Obtaining a contact lens prescription requires a comprehensive eye exam followed by a separate contact lens fitting. This involves specialized measurements of the eye’s surface, often using an instrument called a keratometer or corneal topographer to determine the corneal curvature. A trial lens is then selected based on these measurements, and the eye care professional evaluates how the lens sits, moves, and centers on the eye.
The final contact lens prescription is a medical document that must include specific details not found on a glasses prescription, such as the Base Curve, Diameter, specific lens material or brand, and an expiration date. Federal law, specifically the Fairness to Contact Lens Consumers Act (FCLCA), requires prescribers to automatically provide the patient with a copy of their contact lens prescription once the fitting is complete. This ensures the patient can purchase lenses from any seller.