A common question is whether an eyeglass prescription can be used to order contact lenses. The answer is generally no, as the power values on the two prescriptions will almost certainly be different. This difference exists because eyeglasses sit approximately 12 to 14 millimeters away from the eye, while a contact lens rests directly on the cornea. This physical distance alters the lens’s effective optical power, requiring a mathematical conversion to achieve the same visual clarity.
The Role of Vertex Distance in Power Adjustment
The space between the back surface of an eyeglass lens and the front surface of the cornea is known as the vertex distance. This distance is the primary reason the power, or diopter (D) measurement, in a glasses prescription must be adjusted for contact lenses. Because the contact lens sits directly on the eye, its vertex distance is considered zero, which changes how light is focused onto the retina.
The need for a power conversion becomes significant primarily for individuals with higher prescriptions, typically those exceeding \(\pm 4.00\) diopters. For lower prescriptions, the change in effective power is often so minimal that the numbers on the glasses and contact prescriptions are practically identical. The eye care professional performs a calculation that accounts for the difference between the standard position of the glasses and the zero position of the contact lens.
This conversion follows a predictable rule based on the type of refractive error being corrected. For a person with myopia, or nearsightedness (indicated by a negative power), the contact lens will require a lower magnitude of minus power than the glasses. Conversely, a person with hyperopia, or farsightedness (indicated by a positive power), will require a slightly higher magnitude of plus power in their contact lenses compared to their spectacle lenses.
Essential Contact Lens Measurements Beyond Power
The power of the lens is only one component of a contact lens prescription, which must also include physical measurements unique to the lens itself. The Base Curve (BC) and the Diameter (DIA) are two essential parameters that are not found on a typical glasses prescription. These measurements are expressed in millimeters and are tailored to the unique anatomy of the individual cornea.
The Base Curve, often ranging between 8.3 and 9.5 mm, measures the curvature of the contact lens’s back surface. This curvature must closely match the shape of the cornea to ensure a proper fit, which prevents the lens from being too tight or too loose on the eye. A base curve that is too flat may cause the lens to move excessively or even fall off, while one that is too steep can restrict tear flow and oxygen exchange, potentially compromising eye health.
The Diameter specifies the overall width of the contact lens, typically falling between 13.0 and 14.5 mm for soft lenses. This measurement dictates how much of the cornea and surrounding area the lens will cover. The correct diameter ensures the lens centers properly over the pupil and allows for adequate movement with each blink, which is necessary for clear vision and comfort. Both the Base Curve and the Diameter are manufacturer-specific, meaning a lens from one brand may fit differently than a lens from another.
The Contact Lens Examination and Fitting Process
Because contact lenses are considered medical devices that rest directly on the eye’s delicate surface, a glasses prescription is insufficient for purchasing them. A specialized contact lens fitting examination is required before a prescription can be issued. This process begins with a comprehensive eye health check and an assessment of the cornea’s shape using an instrument called a keratometer or a topographer.
The eye care professional selects a trial lens based on these measurements, and the patient wears it for a period to evaluate its performance. The lens is checked for proper centration, movement, and coverage of the cornea to confirm that the fit is healthy and comfortable. Follow-up visits are often scheduled to assess how the eye tolerates the lens over time and to finalize the prescription.
The final, complete contact lens prescription will include the power, the Base Curve, the Diameter, and the specific brand name of the lens that was successfully fitted. This process ensures the lens provides the correct vision correction while maintaining the long-term health of the eye. The prescription is valid for a limited time, which encourages regular eye health check-ups and reassessment of the fit.