For bifocals, the most common type is the flattop 28 D-style near-vision reading segment lens. What that means is that the top of the bifocal segment lens is flat and the bottom is curved, making the bifocal lens appear like the capital letter D laid on its side. It’s called “flattop 28” because the widest part of the bifocal segment lens is 28 millimeters (mm) across.
One of the bifocal lenses is the 1.50 standard-index bifocal lens in CR-39 plastic. It’s just like the 1.50 standard-index single-vision lens, except it has the bifocal segment. This lens is good for SPHs of -3.00/+1.50 or lower with CYLs of +/- 6.00 and an ADD of +3.50 or lower.
The bifocal segment line is located 2 mm below the center of the lens. So if you get a lens that is 30 mm high, the bifocal segment will be at 13 mm up from the bottom, 2 mm below the 15 mm center line.
A 1.61 high-index aspheric bifocal polymer lens provides a thinner lens for prescriptions with SPHs of -9.00/+6.00 or below with CYLs of +/- 6.00 or lower and an ADD of +3.50 or lower.
As with single-vision high-index lenses, an anti-reflection coating is recommended for high-index bifocal lenses.
Now let’s look at progressive lenses.
Progressive (no-line multifocal) glasses follow the same pattern as the single-vision lenses in terms of the index of the distance portion of the lens. They deviate a bit from the bifocals regarding the near-vision reading segment. The progressive lenses in the style called “free-form” has a reading corridor of approximately 14 mm, roughly half the width of the bifocal lens.
The 1.50 standard-index progressive CR-39 plastic lens covers SPHs of -2.00/+1.00 or lower with CYLs of +/- 6.00 or lower, and an ADD power of +3.00 or lower. The 1.57 mid-index progressive polymer lens covers SPHs of -4.00/+2.00 or lower, CYLs of +/- 6.00 or lower, and an ADD power of +3.50 or lower.
The 1.53 mid-index Trivex progressive lens has, like the single-vision Trivex lens, the highest impact resistance of all the lenses we carry. Also like the single-vision Trivex lens, this lens cannot be tinted. It’s a little thinner than the standard-index 1.50 index lens. It covers SPHs of -4.00/+2.00 or lower, CYLs of +/-4.00 or lower, and an ADD power of +3.00 or lower. It’s recommended for rimless and sports eyeglasses, but unlike the single-vision Trivex lens, it’s not recommended for children, who are prescribed multifocal glasses only in rare instances.
The features of the next progressive lenses, the 1.59 mid-index pure polycarbonate progressive lens, and the high-index 1.61 and 1.67 lenses, follow the same pattern as the single-vision lenses. The 1.59 progressive lens, which, like the Trivex lens, cannot be tinted (although it can be ordered as polarized, glare-reducing sunglasses or with photochromic, “auto-tinting” lenses), covers SPHs of -4.00/+2.00 or lower, CYLs of +/- 4.00 or lower, and an ADD of +3.00 or lower.
The high-index 1.61 progressive polymer lens covers SPHs of -6.00/+3.00 or lower, CYLs of +/- 6.00 or lower, and an ADD of +3.00 or lower. The high-index 1.67 progressive polymer lens covers SPHs of -10.00/+8.00 or lower, CYLs of +/- 6.00 or lower, and an ADD of +3.00 or lower. As with single-vision lenses, anti-reflection coatings are recommended for eyeglasses with high-index lenses.
Now you know all the features of all the prescription lenses.