The strongest eyeglass lenses are high-index materials, typically with an index of refraction around 1.74. These extra-thin lenses can correct prescriptions up to -20.00 or +20.00 diopters.At just 1-2 mm thick, 1.74 index lenses are up to 50% thinner than regular plastic lenses for the same prescription power. This allows them to be mounted in more frame styles with less distortion for those with extreme nearsightedness or farsightedness.