The strength of a contact lens refers to its ability to bend light to focus images correctly onto the retina. This focusing power is measured in units called diopters (D). The number on a prescription indicates how many diopters of correction are needed, with a minus sign (-) signifying correction for nearsightedness (myopia) and a plus sign (+) for farsightedness (hyperopia). The strength of contact lenses available varies widely, driven by standard manufacturing limits and the patient’s individual vision needs.
The Maximum Range for Standard Correction
Most commercially available soft contact lenses correct simple spherical errors. For nearsightedness (minus power), standard soft lenses are commonly manufactured up to approximately -12.00 D. Some mass-produced lines may extend this range to -20.00 D or -25.00 D, depending on the brand and material.
For farsightedness (plus power), the typical range for standard soft lenses is more constrained, often reaching a maximum of +6.00 D to +8.00 D. This limitation is due to how lens thickness affects material properties, which is challenging with high plus powers. When a patient’s error exceeds these standard limits, custom-made soft lenses can be ordered. These custom lenses are available in strengths up to \(\pm 25.00\) D or higher for individuals with extreme refractive errors.
Specialized Lenses for Complex Vision Needs
Lenses designed for complex vision issues introduce additional parameters that limit the overall available power beyond simple spherical correction. Toric lenses correct astigmatism and require three measurements: spherical power, cylinder power (CYL), and axis. The cylinder power measures the astigmatism correction needed and is the most restricted component in soft lenses.
Standard soft toric lenses usually offer cylinder correction up to -2.75 D. For patients with higher astigmatism, custom soft toric lenses are necessary, extending correction to -8.00 D or -10.00 D. These high-power lenses must remain stable on the eye to maintain clear vision, which presents a significant fitting challenge.
Multifocal and bifocal lenses correct presbyopia by introducing “Add Power” (ADD) to enhance near vision. This Add Power is the magnifying boost placed in the central or peripheral zones of the lens. The maximum Add Power available in commercial multifocal lenses is typically +2.50 D, though some brands reach +3.00 D. The combined optical zones in these designs can compromise clarity, especially in low-light conditions, limiting the maximum Add Power.
Physical and Physiological Constraints on Power
The physical dimensions of a contact lens impose the ultimate limit on prescription strength. High minus power lenses (nearsightedness) are thick at the edges, while high plus power lenses (farsightedness) are thickest in the center. This variation in thickness creates a challenge for the health of the cornea.
The cornea is avascular and relies on oxygen absorbed from the atmosphere through the tear film. Any lens covering the cornea must allow sufficient oxygen to pass through, a property measured as oxygen transmissibility (Dk/t). Thicker lenses reduce this oxygen flow, which is particularly problematic for high plus lenses due to their thick center.
Reduced oxygen transmissibility increases the risk of eye health issues, such as corneal swelling or the growth of new blood vessels into the cornea. This physiological requirement for oxygen sets a limit on the central thickness of a lens, constraining the maximum achievable plus power. When a refractive error is too extreme for a contact lens to ensure adequate oxygen flow, alternative options like highly powered eyeglasses or intraocular lens implants become the viable solutions.