High index lenses are engineered materials designed to be thinner and lighter than standard plastic or glass lenses, making them a popular choice for individuals with strong prescriptions. This specialized design allows the lens to maintain the necessary vision correction power without the bulkiness associated with high-power lenses. While they offer significant aesthetic and comfort benefits, high index lenses can cause distortion, particularly in the peripheral view. This potential for visual side effects is a direct result of the optical properties necessary to achieve their slim profile, but these effects are often manageable or minimized through modern lens technology and proper fitting.
Understanding High Index Materials
The ability of high index lenses to be thin is due to their higher refractive index, a measurement of how efficiently the material bends light. A higher refractive index bends light more sharply, meaning less material thickness is needed to achieve the required corrective power. Standard plastic lenses have a refractive index near 1.50, while high index lenses typically start at 1.60.
The trade-off for this increased efficiency is the Abbe value, which measures a material’s light dispersion. Generally, as the refractive index of a lens material increases, its Abbe value decreases. A lower Abbe value means the material scatters light more, which is the underlying cause of color separation or “color fringing.”
This dispersion causes different wavelengths of light to be focused at slightly different points, especially away from the lens center. High index materials have a lower Abbe value, increasing the potential for visual disturbances compared to standard plastic.
Recognizing Specific Visual Effects
The lower Abbe value in high index materials results in a specific visual disturbance known as chromatic aberration. This effect is perceived as color fringing or faint colored halos around objects, especially noticeable when looking at high-contrast items. Chromatic aberration is most apparent when the eye looks away from the optical center of the lens, moving toward the edges.
Another type of distortion commonly experienced is peripheral distortion, often described as the “fishbowl” or “pincushion” effect. This occurs when straight lines in the wearer’s peripheral vision appear to curve outward or inward. This effect is compounded by the flatter curvature of high-power lenses, which is necessary to keep them thin.
The perception of this distortion can be disruptive in daily activities that require a wide field of view, such as driving. The further the line of sight moves from the center, the more pronounced the curvature and color fringing become. The extent of these effects is closely tied to the strength of the wearer’s prescription.
Strategies for Clearer Vision
Several modern optical strategies can significantly mitigate the distortion issues associated with high index lenses. One effective solution is the use of specialized lens designs, such as aspheric or atoric surfaces. These designs flatten the lens curvature toward the edges, which reduces magnification and the peripheral distortion effects common in strong prescriptions.
Choosing the right frame is also important to minimize distortion. Selecting smaller, rounder frames helps ensure the eye is looking through the clearest, most central part of the lens more often. This minimizes viewing through the extreme edges where both chromatic and peripheral distortion are most prevalent.
Proper fitting is another factor, including ensuring the correct vertex distance, the space between the eye and the back surface of the lens. An optician must accurately measure and adjust the frame to ensure the optical center aligns precisely with the pupil. Finally, applying an anti-reflective (AR) coating is recommended for high index lenses, as it reduces internal reflections that can exacerbate the visual appearance of chromatic aberration.