No, a contact lens prescription is not the same as a glasses prescription. While both contain the corrective power needed for clear vision, they differ significantly in two primary areas: the numerical power of the lens and the physical measurements required for a proper fit. The distinct physical positioning of the two types of lenses—on the eye versus a short distance away—means a single set of measurements cannot be used for both. This difference is rooted in the principles of optics.
The Core Difference: Vertex Distance
The main optical reason for the difference in power is a concept called vertex distance. This is the physical space between the back surface of a glasses lens and the front surface of the cornea. Glasses typically sit about 12 to 14 millimeters away from the eye, creating a measurable vertex distance that affects the lens’s effective power.
A contact lens rests directly on the cornea, meaning its vertex distance is effectively zero. This shift changes how light rays are refracted before they enter the eye. For vision to be equally clear, the numerical power of the corrective lens must be mathematically adjusted to compensate for this positional difference.
This power adjustment is particularly noticeable in higher prescriptions, generally those stronger than plus or minus 4.00 diopters. For stronger powers, the conversion is necessary to ensure the light focuses correctly on the retina. For example, a strong nearsighted prescription may result in a weaker contact lens power after the vertex distance conversion.
Measurements Unique to Contact Lenses
Beyond the power difference, a contact lens prescription must include two unique measurements entirely absent from a glasses prescription: Base Curve (BC) and Diameter (DIA). The Base Curve specifies the curvature of the lens’s back surface, determining how the lens rests on the natural curve of the cornea. The Diameter indicates the overall width of the lens from edge to edge.
Both are essential for comfort and ocular health. A lens that is too flat or too steep can cause irritation, prevent proper tear exchange, or lead to blurry vision. These fit measurements ensure the lens is not too tight, which restricts oxygen, nor too loose, which causes excessive movement. Since corneal shape is unique, these specific measurements are determined during a contact lens fitting.
Why a Separate Contact Lens Exam is Mandatory
A contact lens exam is an additional, separate procedure required to determine the correct power and fit for lenses that sit directly on the eye. This process involves the eye care professional first calculating the correct numerical power by adjusting the glasses prescription for the zero vertex distance. They then use specialized instruments to measure the curvature of the cornea to select the appropriate Base Curve and Diameter for the lens.
The exam also includes a trial fit where the eye doctor observes the lens’s movement on the eye, checking for proper alignment and ensuring it allows for sufficient tear flow. This assessment is necessary to confirm the lens is not causing any physiological issues, such as irritation or poor oxygenation, which are direct risks associated with a poorly fitted lens. Because contact lenses are classified as medical devices, federal law mandates a valid, specific prescription for their purchase.
This law requires eye care providers to automatically give the patient a copy of their contact lens prescription after the fitting is complete. The prescription must contain the power, Base Curve, Diameter, and a specific expiration date, which is typically one year, ensuring the patient’s eyes are checked regularly. Therefore, due to the optical conversion and the necessity of physical fit measurements, a separate contact lens exam and prescription are mandatory for safe and legal lens use.