You cannot directly use your glasses prescription to purchase contact lenses, even though both aim to correct vision. A contact lens prescription requires specific measurements and adjustments that a glasses prescription does not include. Contact lenses are classified as medical devices that sit directly on the eye, making a proper, unique fit mandatory for eye health and clear vision. Attempting to use a glasses prescription can lead to discomfort, blurred vision, and potentially serious complications like corneal damage.
The Crucial Difference in Distance
The fundamental difference between glasses and contacts is the distance from the eye, known as vertex distance. Eyeglass lenses sit approximately 12 to 14 millimeters away from the cornea. This fixed gap is accounted for when determining the power of the glasses lens.
Contact lenses rest directly on the tear film of the cornea, meaning the vertex distance is zero. This change in position alters how light is refracted before it reaches the retina, necessitating a different effective lens power. For example, a person with a minus prescription for nearsightedness generally requires a slightly weaker contact lens power compared to their glasses.
For prescriptions less than plus or minus 4.00 diopters, the difference in required power is often negligible. However, as the prescription strength increases, the required power adjustment becomes progressively more significant, resulting in inaccurate vision correction and eye strain if unadjusted.
Contact Lens Specific Measurements
A glasses prescription only specifies optical power, but a contact lens prescription must also include physical parameters to ensure a safe and proper fit. These parameters are unique to contact lenses and are absent from a standard glasses prescription. The two primary physical measurements are the Base Curve (BC) and the Diameter (DIA).
Base Curve (BC)
The Base Curve measures the curvature of the back surface of the contact lens, which is the side that rests directly on the cornea. This measurement, typically expressed in millimeters, must closely match the curvature of the individual’s eye to prevent the lens from being too tight or too loose. An incorrect base curve can lead to discomfort, blurry vision, and interfere with the flow of tears and oxygen exchange.
Diameter (DIA)
The Diameter is the total width of the contact lens, measured in millimeters. For soft lenses, the diameter usually ranges between 13.80 and 14.50 mm and ensures the lens correctly covers the cornea. If the diameter is too small or too large, the lens may move excessively or cause irritation and abrasion to the eye.
Adjusting the Prescription Power
Converting the power values from a glasses prescription to a contact lens prescription involves a mathematical calculation called vertex compensation. This procedure accounts for the elimination of the vertex distance when the lens moves from a position in front of the eye to resting directly on the cornea. Eye care professionals use a specific formula to translate the sphere and cylinder values of the glasses prescription into the appropriate contact lens power.
For highly nearsighted individuals (minus power), the contact lens power will be numerically lower than the glasses prescription. Conversely, for highly farsighted individuals (plus power), the contact lens power will be numerically higher. For example, a spectacle power of -6.00 diopters may convert to an approximate contact lens power of -5.50 diopters.
If a person has astigmatism, the cylinder and axis values must also be converted, which is a more complex process. Contact lenses for astigmatism, known as toric lenses, are not available in the same range of precise axis increments as glasses. This requires the doctor to select the best available power and orientation, and the final power requires an on-eye assessment.
Getting Your Contact Lens Prescription
Obtaining a valid contact lens prescription requires a specific contact lens exam and fitting performed by an eye care professional. This is distinct from a routine eye exam for glasses, as the fitting assesses eye health for contact lens wear and takes the necessary physical measurements. The examination includes measuring the curvature of your cornea, often with a specialized instrument, to determine the correct Base Curve.
Following initial measurements, the eye doctor provides trial lenses to wear. This trial period allows the professional to evaluate how the lens fits and moves on the eye, check for proper tear exchange, and confirm the vision is clear and comfortable. A follow-up appointment is typically scheduled to ensure the eyes are adjusting well to the new lenses.
Upon successful completion of the fitting and evaluation, the eye care professional issues a final, legal contact lens prescription. This document includes:
- The necessary optical power.
- The Base Curve.
- The Diameter.
- The specific brand or material of the lens determined to be the best fit.
The prescription is legally required for purchasing contact lenses, as they are regulated medical devices.