High index lenses are engineered to be thinner and lighter than standard lenses, primarily benefiting individuals with strong prescriptions. The term “high index” refers to the material’s ability to refract light more sharply than traditional plastic, allowing the lens to correct significant vision errors using less physical material. Whether these specialized lenses reduce or potentially introduce visual distortion depends on both the material properties and the final lens design.
Understanding Refractive Index and Lens Thickness
The refractive index is a number representing how quickly light travels through a lens material, directly indicating the material’s light-bending power. Standard plastic lenses (CR-39) have an index of approximately 1.50, while high index materials range from about 1.53 to 1.74. A higher index means the lens material can achieve the same prescription power with a flatter and thinner profile, potentially making a 1.74 index lens up to 50 percent thinner than a 1.50 index lens.
This reduction in thickness is a major benefit for both comfort and aesthetics. For high prescriptions causing thick lens edges (nearsightedness), high index material greatly reduces the noticeable “bug-eye” effect. Conversely, for strong farsighted prescriptions that have a thicker center, high index lenses minimize the magnification effect on the wearer’s eyes.
How Lens Design Corrects Geometric Distortion
While the high index material itself reduces thickness, the lens’s final shape is what determines its geometric, or optical, distortion. Traditional lens designs, called spherical lenses, have a uniform curve across the entire surface, like a section of a ball. In strong prescriptions, this spherical shape causes a significant increase in peripheral distortion, known as pincushion or barrel effects, where straight lines appear to curve outward or inward when looking away from the lens center.
To counteract this, high index lenses are frequently manufactured using an aspheric design, which incorporates a complex, non-uniform curve. Unlike the spherical design, the aspheric lens curvature gradually flattens toward the edges. This flatter profile is necessary to maintain optical clarity across the entire lens surface and reduce the distortion that would otherwise be exaggerated by the higher refractive power. The combination of a high index material and an aspheric design effectively minimizes geometric distortion, providing a sharper image across a wider field of view.
Some advanced options, known as double aspheric lenses, apply this complex curvature to both the front and back surfaces of the lens. This dual-surface optimization maximizes the reduction of peripheral distortion and results in the thinnest, flattest possible lens profile for extremely high prescriptions. The greatest reduction in visual distortion from a high index lens comes not from the material alone, but from the deliberate incorporation of aspheric or atoric lens geometry.
The Impact of High Index Material on Color Distortion
A separate type of distortion is chromatic aberration, which manifests as color fringing or faint rainbow halos around high-contrast objects. This visual effect occurs because the lens material disperses white light into its component colors, failing to focus all wavelengths at the same point. This phenomenon is quantified by the Abbe value, where a higher number indicates less color dispersion and better optical clarity.
A trade-off inherent in lens manufacturing is that as the refractive index of a material increases, its Abbe value generally decreases. Standard plastic lenses (1.50 index) have a high Abbe value of around 58, indicating minimal color distortion. In contrast, high index plastics (1.67 to 1.74) have lower Abbe values, often ranging from 32 to 39, making them more prone to noticeable chromatic aberration.
The color fringing associated with a lower Abbe value is a material limitation, distinct from the geometric warping corrected by aspheric design. This distortion is typically most apparent when looking through the edges of the lens. Selecting a high index material involves balancing the aesthetic benefits of a thinner lens against the potential for increased color distortion, though many wearers find the visual impact to be minor.