When seeking vision correction, a common question arises: “Is your contact prescription the same as your glasses?” The clear answer is no. While both glasses and contact lenses effectively correct vision, they achieve this through distinct methods, requiring separate and specific prescriptions.
Fundamental Optical Differences
The primary reason for the difference in prescriptions stems from how each corrective lens interacts with the eye. Glasses sit at a distance from the eye, typically 12 to 14 millimeters, on the bridge of the nose. This separation, known as vertex distance, influences light refraction. In contrast, contact lenses rest directly on the cornea, eliminating this distance. This difference in positioning means the optical power required for clear vision changes between the two forms of correction; for example, a spectacle prescription of -5.00 diopters might translate to a slightly weaker contact lens power like -4.75 diopters.
Specific Prescription Adjustments
Beyond the power adjustment for the sphere, contact lens prescriptions include several unique parameters. The base curve (BC) measures the curvature of the back surface of the lens, which must precisely match the wearer’s cornea for a proper and comfortable fit (typically 8.3-9.0 mm for soft lenses). The diameter (DIA) specifies the overall width of the lens, ensuring correct coverage without being too loose or too tight (common soft lens diameters range from 13.5-14.5 mm). For individuals with astigmatism, toric contact lenses are prescribed. These include cylinder and axis values to correct irregular curvature and have different refractive powers across their surface to properly focus light.
The Contact Lens Fitting Process
Obtaining a contact lens prescription involves a detailed fitting process that extends beyond a standard eye examination for glasses, beginning with a comprehensive eye exam to assess overall eye health and visual acuity. Specialized measurements are then taken, including corneal curvature (using a keratometer or corneal topography) and sometimes pupil and iris diameter. The eye care professional evaluates tear film quality to ensure the eyes can tolerate contact lens wear. Following these measurements, trial lenses are selected, and their fit and comfort are evaluated on the eye. Follow-up appointments confirm proper vision, comfort, and eye health, ensuring the chosen lens is suitable for long-term wear.
Risks of Using the Wrong Prescription
Using an incorrect contact lens prescription or attempting to use a glasses prescription for contacts carries several potential hazards. Wearing lenses with the wrong power can lead to blurred or distorted vision, headaches, and eye strain. An improperly fitted lens, either too tight or too loose, can cause significant discomfort, irritation, and even corneal abrasions. Incorrectly fitted lenses may also reduce oxygen flow to the cornea, increasing the risk of eye infections and potentially leading to permanent eye damage. Therefore, a precise, professionally determined contact lens prescription is important for clear vision and maintaining ocular health.