A contact lens prescription is not the same as a glasses prescription, and they are not interchangeable. While both correct the same vision problem, the lens power, measurements, and legal requirements are distinct for each. This difference stems from the fact that a contact lens sits directly on the eye’s surface, whereas a glasses lens is positioned a short distance away. This difference in position fundamentally changes the optical power needed for clear vision.
The Role of Vertex Distance in Power Calculation
The numerical power listed on the two prescriptions often differs because of “vertex distance.” This term refers to the space between the back surface of a glasses lens and the front of the eye’s cornea, typically around 12 to 14 millimeters. Contact lenses, in contrast, rest directly on the cornea, resulting in a zero vertex distance. This change in distance impacts the effective power of the lens that the eye perceives.
The optical power required to focus light onto the retina changes when the lens moves closer to the eye. For prescriptions stronger than plus or minus 4.00 diopters, a mathematical adjustment is necessary. This conversion is performed by the eye care professional to compensate for the eliminated vertex distance.
If this calculation is not performed for stronger prescriptions, the vision correction will be inaccurate. For example, a -6.00 diopter glasses prescription might convert to approximately a -5.50 diopter contact lens power. A patient with a strong minus prescription will typically require a contact lens power that is less minus than their glasses prescription. This conversion must also be applied independently to both the sphere and cylinder components for prescriptions with astigmatism.
Measurements Unique to Contact Lens Fit
Beyond the difference in optical power, a contact lens prescription must include unique measurements not found on a glasses prescription. These parameters are essential because a contact lens is a regulated medical device that sits directly on the eye. The fit must be precise to ensure comfort, clear vision, and long-term eye health.
A primary measurement is the Base Curve (BC), which defines the curvature of the lens’s back surface in millimeters. This curvature must closely match the natural curvature of the patient’s cornea. If the lens is too flat or too steep, it can cause irritation or restrict oxygen flow. Another measurement is the Diameter (DIA), which specifies the width of the lens from edge to edge, ensuring it covers the appropriate area of the cornea.
The prescription also specifies the exact brand and material of the lens, such as hydrogel or silicone hydrogel. Different lens materials have varying levels of oxygen permeability, which is a major factor in corneal health. The combination of the Base Curve, Diameter, and material type is determined through specific corneal mapping and a trial lens fitting.
The Difference in Examination and Validity
Obtaining a contact lens prescription requires a specific fitting examination separate from a standard eye exam for glasses. This fitting involves the eye care professional placing a trial lens on the eye to evaluate its movement, position, and overall fit. The doctor uses specialized equipment to assess how the lens interacts with the cornea and tear film, ensuring the lens rests and moves correctly with each blink.
Contact lens prescriptions also have a distinct legal lifespan compared to glasses prescriptions. Under the Federal Trade Commission’s (FTC) Contact Lens Rule, prescribers must provide patients with a copy of their prescription after the fitting is complete. The Rule sets a minimum expiration date of one year for a contact lens prescription, unless a medical reason requires a shorter period. This shorter validity period acknowledges the higher risk of complications, such as infection or injury, associated with improperly fitted or worn lenses.