The question of whether a glasses prescription can be used to order contact lenses is a common one. The answer is generally no; a prescription for eyeglasses is not sufficient for purchasing contact lenses, even if the power seems similar. Contact lenses are classified as medical devices and require a separate, specialized examination and fitting process to ensure both proper vision correction and the health of the eye. This necessity is due to the fundamental difference in how each lens type sits on and interacts with the eye’s surface.
Why Glasses and Contacts Require Different Power Measurements
The optical power required for clear vision often changes significantly between glasses and contacts because of a concept known as vertex distance. This distance refers to the space between the back surface of an eyeglass lens and the front of the cornea, typically about 12 to 14 millimeters. Because of this gap, the light is refracted before it reaches the eye’s focusing system.
In contrast, a contact lens sits directly on the cornea, meaning the vertex distance is zero. This difference in position changes the effective power of the lens needed to correct vision. For lower prescriptions, the difference is negligible, but for stronger prescriptions, adjustment is necessary.
Eye care professionals must perform a mathematical calculation, called vertex compensation, to convert the glasses prescription power into the correct contact lens power. For sphere powers stronger than about plus or minus 4.00 diopters, this conversion is especially important for accurate vision correction. For instance, a high minus power for nearsightedness typically becomes a slightly lower minus power in a contact lens prescription.
The Required Contact Lens Exam and Fitting
Obtaining a contact lens prescription requires a specialized fitting procedure that goes beyond a standard eye exam. The process begins with a comprehensive assessment of the eye’s overall health, including an examination of the cornea and surrounding eye surface. This check confirms the eyes can safely tolerate wearing a contact lens without risk of irritation or infection.
Specialized measurements are taken to determine the precise curvature and size of the eye. A device called a keratometer or a corneal topographer is used to map the curvature of the cornea, the clear, front surface of the eye. The doctor will also evaluate the quantity and quality of the tear film, as inadequate tear production can lead to discomfort and make contact lens wear unsuitable.
After the initial measurements, trial lenses are placed on the eye to evaluate the fit, comfort, and movement. The lens must rest gently on the eye, allowing for proper tear exchange and oxygen permeability to the cornea. If the lens is too tight, it can restrict oxygen flow; if it is too loose, it can move excessively and cause blurry vision. Follow-up appointments are often scheduled to confirm the lenses are fitting correctly and comfortably over time.
Understanding Your Contact Lens Prescription Parameters
The final contact lens prescription contains several pieces of information that are not present on a glasses prescription, reflecting the lens’s physical fit on the eye. These parameters ensure the lens rests safely and stably on the cornea.
Base Curve (BC)
The Base Curve, often labeled BC, measures the curvature of the back surface of the lens in millimeters. This number, typically ranging from 8.0 to 9.5 mm for soft lenses, must closely match the curvature of the wearer’s cornea. A lower BC value indicates a steeper curve, while a higher value indicates a flatter curve, ensuring a safe and stable fit.
Diameter (DIA)
The Diameter, or DIA, indicates the overall width of the lens from edge to edge, also in millimeters. For soft lenses, the diameter usually falls between 13.0 and 14.5 mm and is necessary to ensure the lens fully covers the cornea. An incorrect diameter can result in the lens being too loose or too tight, leading to discomfort or damage to the eye’s surface.
Brand, Material, and Expiration
The prescription also specifies the exact Brand Name and Material, as different lens types possess unique properties regarding water content and oxygen transmission. Finally, a contact lens prescription includes an Expiration Date, typically one year from the date of the exam. After this date, a new fitting is legally required.