“Eye power” in the context of contact lenses refers to the specific lens strength required to correct a person’s vision. This measurement is determined by an eye care professional after a comprehensive exam and is a defining characteristic of your contact lens prescription. A contact lens prescription must be highly specific because the lens rests directly on the eye’s surface. The lens power ensures that light focuses precisely onto the retina for clear and sharp vision.
Decoding the Power Measurement
The optical strength of a contact lens is quantified using the diopter, often abbreviated as “D.” This measurement, typically listed as “SPH” or “PWR” on a prescription, indicates the degree of refractive correction needed. A negative sign (e.g., -2.00 D) signifies correction for nearsightedness (myopia), where the eye focuses light too soon. Conversely, a positive sign (e.g., +1.50 D) corrects for farsightedness (hyperopia), where the eye focuses light too far back, causing difficulty with near vision.
The Difference Between Glasses and Contact Power
The power number in a contact lens prescription is often different from that in a glasses prescription, even for the same eye. This difference is due to “vertex distance,” the space between the corrective lens and the front of the eye’s cornea. Eyeglasses sit approximately 12 to 14 millimeters away, while a contact lens rests directly on the cornea, making its vertex distance zero. For prescriptions greater than roughly ±4.00 diopters, the power must be mathematically adjusted to compensate for this positional difference. This calculation ensures the contact lens provides the same visual correction as glasses, which is why the two prescriptions are not interchangeable.
Essential Parameters Beyond Power
The contact lens power is only one part of the complete prescription, which also includes measurements for proper fit and specific types of vision correction.
Base Curve (BC) and Diameter (DIA)
The Base Curve (BC), measured in millimeters, defines the curvature of the back surface of the lens. This measurement must match the natural curvature of the cornea. If the BC is too flat or too steep, the lens will not fit correctly, potentially causing discomfort or eye health issues. The Diameter (DIA), also in millimeters, defines the total width of the lens from edge to edge. The DIA ensures the lens provides sufficient coverage over the cornea for a stable and comfortable fit.
Astigmatism Correction
For individuals with astigmatism (an irregular curvature of the cornea), the prescription includes additional values for correction. The Cylinder (CYL) indicates the additional lens power needed, while the Axis (AXIS) specifies the orientation (measured in degrees from 0 to 180) where that power must be placed. These values are exclusive to toric contact lenses.