You’re ordering glasses online.
You enter your prescription and your pupillary distance. (The pupillary distance, abbreviated PD, is the distance from the center of one pupil to the center of the other pupil. It’s as necessary as your prescription to get glasses online.)
The website you’re ordering from reads these items then recommends a lens for your eyeglasses.
How does it know which lens is right for you?
Here’s how. Each optical lens, from standard index to high index, from single vision to multifocal, accommodates a particular prescription range.
Let’s take a look at the various prescription lenses optical retailers offer, the various prescription types, and the prescriptions within those types the lenses cover.
The two broad prescription categories are single vision and multifocal. Multifocal prescriptions comprise bifocals and progressives.
Nearsightedness and farsightedness are corrected according to the numbers in the Sphere (SPH) section of a prescription. Single-vision prescriptions for nearsightedness can be filled up to -20.00, for farsightedness up to +12.00.
If you have an astigmatism, this will be indicated on your prescription by numbers in the Cylinder (CYL) and Axis (sometimes abbreviated “X”) fields. For single-vision prescriptions, meaning those with no numbers in the Near-Vision Reading ADDition section (NV-ADD or just ADD), Prescriptions in the CYL section can be filled up to + or – 6.00. The numbers in the Axis or X section just refer to the angle at which the CYL, an actual cylinder that’s invisible to the wearer, is placed on the lens.
A tiny portion of eyeglasses wearers have prism corrections on their prescription. These can be filled for single-vision (not multifocal) prescriptions up to 5.00 in any base direction (base up, base down, base in, or base out).
For people who have numbers in the NV-ADD section of their prescription and want to get the type of multifocal glasses known as bifocals, which are eyeglasses with a lined reading-addition segment at the bottom of the lens, prescriptions for nearsightedness can be filled up to -9.00 and for farsightedness up to +6.00. You can go up to +3.50 on the NV-ADD.
The type of multifocal eyeglasses known as progressives are glasses with no visible line between the three different focal segments, which are distance (driving and watching TV) vision at the top of the lens, intermediate (computer) vision at the middle of the lens, and near (reading) vision at the bottom of the lens.
Prescriptions can be filled with higher corrections for progressives than for bifocals. Most progressives go up to -10.00 for nearsightedness and up to +8.00 for farsightedness. Just like with bifocals or single-vision glasses, the CYL correction goes up to + or – 6.00. And just like with bifocals, the NV-ADD goes up to +3.50.
Here are the specific single-vision lenses, with the prescriptions they accommodate.
For the mildest of single-vision prescriptions, those ranging from -2.00 to +1.00 or lower on the SPH and +/- 2.00 or lower on the CYL, the recommended lens is the 1.50 standard-index lens.
This lens is made of CR-39 plastic. CR-39 plastic is a polymer (a polymer is a large molecule composed of many repeated subunits, known as monomers) that gets its name because it was the 39th formula of a plastic developed by the Columbia Resins (hence “CR”) project in 1940. The first use of this plastic was to help create glass-reinforced plastic fuel tanks for the B-17 bomber in World War II.
CR-39 plastic has an index of refraction of 1.498, rounded up to 1.50 in the optical world.
Another lens available for single-vision prescriptions is the 1.57 mid-index lens. This lens is often described as “polycarbonate composite,” because in addition to polycarbonate the lens is composed of other polymers and resins that keep the lens thinner than the 1.50 lens. Its range is -4.00 /+2.00 or lower on the SPH and +/- 6.00 or lower on the CYL.
The 1.53 mid-index Trivex lens is a polymer lens that is the most impact-resistant lens available. It covers the same range as the 1.57 mid-index lens: -4.00/+2.00 or lower on the SPH and +/- 6.00 or lower on the CYL. It’s recommended for rimless, children’s, and sports eyeglasses because of its unequaled impact resistance. However, unlike the 1.50 and 1.57 lenses, it cannot be tinted.
If $29.95 is more than you would want to spend on an impact-resistant single-vision lens, the 1.59 pure polycarbonate lens, for $9.00, may be preferable. It covers the same prescription range as the 1.57 polycarbonate composite and 1.53 Trivex lenses, -4.00/+2.00 or lower on the SPH and +/- 6.00 or lower on the CYL. Also, it’s the second most impact-resistant lens available, just down a notch on impact resistance from the 1.53 Trivex lens, making it a good, less-expensive alternative for rimless, children’s, and sports eyeglasses.
However, like the 1.53 Trivex lens, it cannot be tinted, although it is available in glare-reducing polarized sunglasses and photochromic “auto-tinting” lenses, which turn dark in the bright sunlight and become clear again when you are out of the sun.
Occasionally, some people who have rimless or half-rim glasses with 1.59 pure polycarbonate lenses have reported some chromatic aberration, meaning rainbow-like reflections, around the outer edge of the lens.
In addition, the 1.59 pure polycarbonate lens tends to scratch more easily than the 1.50, 1.57, and 1.53 lenses, because it’s a softer lens material.
The 1.61 high-index polymer single-vision lens provides a thinner lens for stronger prescriptions than mid- and standard-index optical lenses do. It is suitable for SPHs of -6.00/+3.00 or lower and, like all the others except for the 1.50 standard-index lens, CYLs that go up to +/- 6.00.
A high-index lens reduces the edge thickness for nearsighted (-) prescriptions and the center thickness for farsighted (+) prescriptions. In addition, it has a flatter, aspheric lens surface, which improves the visual quality the wearer experiences while reducing distortion of their eyes when others are looking at you.
Next comes the 1.67 high-index polymer single-vision lens. It provides a thinner lens for prescriptions than the 1.50 standard-index, 1.53 Trivex, 1.57 mid-index, 1.59 polycarbonate, and the 1.61 high-index lenses. It covers SPHs of -20.00/+10.00 or lower and CYLs of +/- 6.00 or lower.
Just like the 1.61 high-index lens, the 1.67 lens reduces the edge thickness for nearsighted (-) prescriptions and the center thickness for farsighted (+) prescriptions. And just like the 1.61 high-index lens, its aspheric lens surface improves the visual quality the wearer experiences while reducing distortion of the wearer’s eyes as seen by others.
The highest index single-vision plastic lens is the 1.74 high-index polymer lens. It’s for nearsighted (-) single-vision prescriptions only. It provides a thinner lens for SPHs of -8.25 to -10.00 or lower and covers CYLs of +/- 4.00 or lower.
Just like the 1.53 mid-index Trivex and 1.59 polycarbonate lenses, the 1.74 lens cannot be tinted. It’s unavailable as a polarized or photochromic lens. Like the 1.61 and 1.67 high-index lenses, it has an aspheric surface, improving the wearer’s visual quality while reducing distortion of the wearer’s eyes as seen by others.
Anti-reflection coatings are good for any optical lenses, but they are especially recommended if you’re getting high-index lenses. They reduce glare and reflection and allow more light to pass through the lens, improving contrast, which improves visual acuity.
That covers single-vision lenses.
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 with high-index lenses.
Now you know all the features of all the prescription lenses.