An eyeglass prescription with a high number represents the degree of refractive error—the eye’s inability to correctly focus light onto the retina. Corrective lenses use specific power to counteract this error, allowing light to focus properly and create clear vision. Understanding what constitutes a “high” prescription requires knowing how these powers are measured and the common thresholds used by eye care professionals.
How Eye Prescriptions Are Measured
The strength of a corrective lens is quantified in units called Diopters (D). A diopter measures the optical power of a lens, indicating the degree to which it converges or diverges light. The higher the numerical value, the stronger the correction needed for clear vision.
The primary measure of vision correction is the Sphere (SPH) value found on a prescription. A minus sign (-) indicates myopia (nearsightedness), where distant objects appear blurry. A plus sign (+) or no sign indicates hyperopia (farsightedness), where close-up objects are typically blurred. The SPH value represents the uniform lens power required to correct this basic focusing error.
Numerical Definitions of a High Prescription
Prescriptions are categorized into mild, moderate, and high based on the Sphere (SPH) value. A prescription is generally considered high when the correction reaches a significant magnitude. For nearsightedness (myopia), prescriptions of -6.00 D or greater are widely classified as high myopia. Prescriptions exceeding -9.00 D are sometimes referred to as extreme myopia.
For farsightedness (hyperopia), a prescription of +5.00 D or more is commonly considered high. These thresholds indicate a significant refractive error requiring substantial lens power. Astigmatism is corrected by the Cylinder (CYL) value; a cylinder of 2.00 D to 4.00 D is considered high, though the overall strength is often determined by the spherical equivalent.
Practical Considerations for Strong Lenses
High-power prescriptions present unique challenges in manufacturing and wearing eyeglasses. Standard lens materials result in very thick and heavy lenses, often leading to a noticeable “Coke bottle” effect. For nearsightedness, strong lenses are thicker at the edges, while lenses for farsightedness are thicker in the center.
To counteract this, individuals with strong prescriptions are advised to use high-index lenses. These specialized materials bend light more efficiently, allowing for a thinner and lighter lens construction. For prescriptions of +/- 4.00 D and higher, high-index materials are recommended to improve aesthetics and comfort.
A higher index of refraction, such as 1.67 or 1.74, means less material is needed to achieve the required correction, significantly reducing lens thickness. Strong lenses can still cause visual distortions that a wearer must adapt to. These distortions include a mild “fish-eye” effect or minification of the eyes behind the lens, especially noticeable in the periphery.
High Prescriptions and Eye Health Risks
A very high prescription can be associated with increased risks to long-term eye health, particularly high myopia. High myopia involves an eyeball that has grown too long (axial length), causing internal eye structures, including the retina, to become stretched and thinned.
This stretching elevates the risk of several serious, vision-threatening conditions. People with high myopia have a significantly higher risk of developing retinal detachment, myopic macular degeneration, and primary open-angle glaucoma.
Regular, comprehensive eye examinations are crucial for anyone with a high prescription, especially -6.00 D or more. The focus shifts from merely correcting vision to actively monitoring the eye’s internal structures for early signs of complications. Early detection and treatment of conditions like retinal tears or glaucoma are essential for preserving vision.