Glasses and contact lens prescriptions are not the same. Although both correct vision to the same clarity, the optical power must differ due to the device’s position relative to the eye. Glasses sit at a measurable distance from the cornea, while a contact lens rests directly on the eye’s surface. This physical difference requires a mathematical adjustment to the lens power for an identical visual outcome. Contact lens prescriptions also include unique physical specifications necessary for a correct and safe fit.
The Critical Role of Vertex Distance
The difference in power is primarily due to vertex distance, which is the space between the back surface of a lens and the front surface of the cornea. Eyeglass lenses typically sit about 10 to 14 millimeters away from the eye, creating a small but significant separation. In contrast, a contact lens is placed directly on the eye, making its vertex distance effectively zero. This change in distance fundamentally alters the effective power of the lens as light travels through it to reach the eye’s focusing system.
The power difference becomes significant for stronger prescriptions, generally those above ±4.00 diopters (D). For nearsighted patients (myopic, minus power), the contact lens power will be slightly less negative than the glasses prescription. For example, a spectacle prescription of -6.00 D converts to a contact lens power of around -5.50 D.
Conversely, for a person who is farsighted (hyperopic, indicated by a plus power), the contact lens requires a slightly more positive power than the glasses prescription. Eye care professionals use a specific mathematical formula to calculate this vertex-corrected power. This adjustment ensures the light is bent by the correct amount, compensating for the change in lens position and providing the same clear vision as the glasses.
Essential Measurements Unique to Contact Lenses
Beyond the optical power, a contact lens prescription contains unique physical measurements that are completely absent from a glasses prescription. These parameters dictate how the lens physically interacts with the sensitive surface of the eye. The two most important measurements are the Base Curve (BC) and the Diameter (DIA), both measured in millimeters.
Base Curve (BC)
The Base Curve (BC) measures the curvature of the back surface of the contact lens. This measurement must be tailored to match the unique curvature of the patient’s cornea. A typical BC for a soft contact lens ranges from about 8.0 mm to 9.5 mm. A lens that is too flat or too steep can lead to discomfort, blurry vision, or damage to the corneal tissue.
Diameter (DIA)
The Diameter (DIA) is the total width of the lens, ensuring it covers the cornea adequately. For soft lenses, the diameter usually falls between 13.0 mm and 15.0 mm. An improperly sized diameter can cause the lens to move too much or too little, leading to irritation or oxygen deprivation. The prescription also specifies the lens material and brand, which affect oxygen permeability and the recommended wear schedule.
The Necessity of a Dedicated Contact Lens Exam
Contact lenses are medical devices that sit directly on the eye, making a dedicated fitting and examination mandatory and separate from a standard eye exam. This specialized appointment ensures the lenses are not harming the ocular surface. The eye doctor must evaluate the lens movement and positioning to confirm a proper fit, which is crucial for comfort and long-term eye health.
During the fitting, the professional assesses the health of the cornea and surrounding tissues, looking for issues like dryness, irritation, or oxygen deprivation. An ill-fitting lens can lead to serious complications, including corneal abrasions or infections. Since eyes and lens technologies change, the contact lens prescription must be renewed regularly, typically annually, to maintain safety and optimal vision.