The power of a contact lens refers to the measurement of its strength, which is the degree to which the lens must bend incoming light to focus it onto the retina at the back of the eye. This measurement dictates the necessary correction required to counteract a person’s specific refractive error. The power number on a prescription ensures that light rays converge or diverge correctly, allowing the wearer to achieve clear, sharp vision.
Understanding the Diopter and Lens Strength
The standard unit of measurement for lens power, both in glasses and contact lenses, is the diopter, abbreviated as ‘D’. This unit quantifies the ability of the lens to redirect light. The diopter value is the number you see on a prescription under the heading PWR or SPH (power or sphere).
The magnitude of this number indicates the degree of correction needed. A higher numerical value signifies a stronger lens is required to correct a more significant refractive error, regardless of whether the sign is plus or minus. For example, a prescription of -5.00 D represents a stronger correction than a -2.00 D prescription.
Contact lens powers are available in small increments, often increasing or decreasing in steps of 0.25 diopters. This fine-tuning allows eye care professionals to select a lens strength that provides the clearest possible vision for the wearer. Common powers range from about -6.00 D to +6.00 D, though stronger corrections are available in some lens types.
Positive and Negative Power: Correcting Common Vision Errors
The sign preceding the diopter number (plus (+) or minus (-)) indicates the type of vision problem being corrected. This sign relates to whether the lens must converge or diverge light to place the focus correctly on the retina.
A negative power, indicated by a minus sign, corrects for myopia (nearsightedness). In a myopic eye, light focuses in front of the retina because the eyeball is slightly longer than average. The minus-powered lens is concave and works by causing light rays to diverge. This effectively shifts the focal point backward onto the retina for clear distance vision.
A positive power, marked by a plus sign, corrects hyperopia (farsightedness). Hyperopia occurs when the eye is shorter than average, causing light to focus behind the retina. The plus-powered lens is convex and functions by converging light rays. This brings the focal point forward onto the retina to improve close-up vision.
Why Contact Lens Power May Differ From Glasses Power
A common point of confusion is why a contact lens prescription might not exactly match a person’s glasses prescription. This difference is explained by vertex distance, which is the physical space between the back surface of a spectacle lens and the front of the eye’s cornea.
Standard glasses sit approximately 12 to 14 millimeters away from the eye. In contrast, a contact lens rests directly on the surface of the eye, making the vertex distance essentially zero. This shift in distance changes the effective optical power of the lens that the eye receives.
For high prescriptions, typically those above +/- 4.00 diopters, this spatial difference becomes optically significant and requires a mathematical adjustment. An eye care professional must recalculate the power to ensure the contact lens provides the same visual correction as the glasses. For minus powers, the contact lens will require slightly less minus power than the glasses, while for plus powers, it will require slightly more plus power.
How Your Lens Power is Determined
Determining the correct contact lens power begins with a comprehensive eye examination. A key part of this exam is the refraction test, which objectively and subjectively measures the eye’s refractive error. The subjective portion of the refraction is what most people recognize as the “which is better, one or two?” test.
During the refraction, the eye care professional uses a phoropter, a device containing a series of lenses that can be quickly changed. The patient looks through the phoropter at a distant eye chart while the doctor systematically adjusts the lens powers in small steps, often 0.25 D. This process continues until the sharpest vision is achieved, determining the sphere, cylinder, and axis values.
The final contact lens power is derived from the results of the refraction, but it is not a direct transfer of the glasses prescription. The professional must apply a vertex distance calculation to the glasses power. They also consider other factors specific to contact lenses, such as how the lens fits and moves on the eye’s surface, to deliver optimal clarity and comfort.