You are ordering glasses online.
You enter your prescription and your pupillary distance. (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 when buying glasses online.)
The website you order from reads these articles and then recommends a lens for your glasses.
How do you know which lens is right for you?
That is how. Each optical lens, from standard index to high index, single vision to multifocal, is tailored to a particular prescription range.
Let’s take a look at the various prescription lenses offered by optical retailers, the various prescription types, and the prescriptions within those types that the lenses cover.
The two broad prescription categories are monofocal and multifocal. Multifocal prescriptions, including bifocal and progressive.
Nearsightedness and farsightedness are corrected according to the numbers in the Sphere (SPH) section of a prescription. Single vision prescriptions for myopia can be filled up to -20.00, for hyperopia up to +12.00.
If you have astigmatism, this will be indicated on your prescription by numbers in the Cylinder (CYL) and Axis (sometimes abbreviated as “X”) fields. For single vision prescriptions, meaning those that do not have numbers in the Near Vision Reading ADD 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 only refer to the angle at which the CYL, an actual cylinder that is invisible to the user, is placed on the lens.
A small portion of eyeglass wearers have prism corrections on their prescription. These can be filled for single vision (non-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 glasses with a lined reading addition segment on the bottom of the lens, prescriptions can be filled for myopia. up to -9.00 and for hyperopia up to +6.00. It can go up to +3.50 in the NV-ADD.
The type of multifocal glasses known as progressives are glasses without a visible line between the three different focal segments, which are distance vision (driving and watching TV) at the top of the lens, intermediate vision (computer) at the middle of the lens. lens, and near vision (reading) 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. As with bifocal or single vision lenses, the CYL correction goes up to + or – 6.00. And just like with bifocals, the NV-ADD goes up to +3.50.
These are the specific monofocal lenses, with the prescriptions that adapt.
For the lightest single vision prescriptions, those ranging from -2.00 to +1.00 or less in the SPH and +/- 2.00 or less in the CYL, the recommended lens is the standard 1.50 index lens.
This lens is made from CR-39 plastic. CR-39 plastic is a polymer (a polymer is a large molecule made up of many repeating subunits, known as monomers) that gets its name because it was the 39th formula of a plastic developed by the Columbia Resins project (hence “CR” ) 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 a refractive index of 1.498, rounded to 1.50 in the optical world.
Another lens available for single vision prescriptions is the 1.57 medium index lens. This lens is often described as a “polycarbonate composite” because in addition to polycarbonate, the lens is made up of other polymers and resins that keep the lens thinner than the 1.50 lens. Its range is -4.00 /+2.00 or less in SPH and +/- 6.00 or less in CYL.
The 1.53 Medium Index Trivex Lens is a polymer lens that is the most impact resistant lens available. Covers the same range as the 1.57 mid-index lens: -4.00/+2.00 or less in SPH and +/- 6.00 or less in CYL. It is recommended for rimless, children’s and sports glasses due to its unmatched impact resistance. However, unlike the 1.50 and 1.57 lenses, it cannot be tinted.
If $29.95 is more than you’d like to spend on an impact-resistant single vision lens, the 1.59 pure polycarbonate lens, for $9.00, may be preferable. Covers the same prescription range as 1.57 polycarbonate composite lenses and 1.53 Trivex lenses, -4.00/+2.00 or less in SPH and +/- 6.00 or less in CYL. Plus, it’s the second most impact resistant lens available, just one notch below the impact resistance of the Trivex 1.53 lens, making it a good, less expensive alternative to rimless, kids’ and sports eyewear. .
However, like the Trivex 1.53 lens, it cannot be tinted, although it is available in glare-reducing polarized sunglasses and “self-tinting” photochromic lenses, which darken in sunlight and become clear when it is not exposed to the sun. .
Occasionally, some people who have rimless or half-rim glasses with 1.59 pure polycarbonate lenses have reported some chromatic aberration, that is, rainbow-like reflections, around the outer edge of the lens.
Also, the 1.59 pure polycarbonate lens tends to scratch more easily than the 1.50, 1.57 and 1.53 lenses, because it is a softer lens material.
The 1.61 high index polymer single vision lens provides a thinner lens for stronger prescriptions than standard and medium index optical lenses. It is suitable for SPH of -6.00/+3.00 or less and, like all other lenses except the standard 1.50 index, CYL ranging up to +/- 6.00.
A high index lens reduces edge thickness for nearsighted (-) prescriptions and center thickness for farsighted (+) prescriptions. Additionally, it has a flatter aspherical lens surface, which improves the visual quality experienced by the wearer while reducing distortion to their eyes when others look at them.
Next comes the 1.67 high index polymer single vision lens. Provides a thinner prescription lens than standard index 1.50, Trivex 1.53, mid index 1.57, polycarbonate 1.59 and high index 1.61 lenses. Covers SPH of -20.00/+10.00 or less and CYL of +/- 6.00 or less.
Like the 1.61 High Index lens, the 1.67 lens reduces edge thickness for nearsighted (-) prescriptions and center thickness for farsighted (+) prescriptions. And like the 1.61 High Index lens, the aspherical lens surface enhances the visual quality experienced by the wearer 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 is only for myopic (-) single vision recipes. Provides a thinner lens for SPH of -8.25 to -10.00 or less and covers CYL of +/- 4.00 or less.
Like the 1.53 medium index Trivex lenses and the 1.59 polycarbonate lenses, the 1.74 lens cannot be tinted. It is not available as a polarized or photochromic lens. Like the 1.61 and 1.67 high index lenses, it has an aspherical surface, which improves the wearer’s visual quality and reduces distortion of the wearer’s eyes as seen by others.
Anti-reflective coatings are good for any optical lens, but are especially recommended if you are going to get 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 flat top 28D 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 look like the capital letter D laid on its side. It is called “flattop 28” because the widest part of the bifocal segment lens is 28 millimeters (mm) wide.
One of the bifocal lenses is the standard 1.50 index bifocal lens in CR-39 plastic. It’s like the standard 1.50 index single vision lens, except it has the bifocal segment. This lens is good for SPH of -3.00/+1.50 or less with a CYL of +/- 6.00 and an ADD of +3.50 or less.
The bifocal segment line is located 2mm below the center of the lens. So if you get a 30mm tall lens, the bifocal segment will be 13mm above the bottom, 2mm below the 15mm center line.
A 1.61 high index aspheric bifocal polymer lens provides a thinner lens for prescriptions with an SPH of -9.00/+6.00 or less with a CYL of +/- 6.00 or less and an ADD of + 3.50 or less.
As with high index single vision lenses, an anti-reflective coating is recommended for high index bifocals.
Now let’s take a look at progressive lenses.
Progressive (non-lined multifocal) glasses follow the same pattern as single vision lenses in terms of the index of the distance portion of the lens. They deviate a bit from bifocals when it comes to the near reading segment. So-called “freeform” style progressive lenses have a reading corridor of about 14mm, about half the width of the bifocal.
1.50 standard index progressive CR-39 plastic lens covers SPH of -2.00/+1.00 or less with CYL of +/- 6.00 or less, and an ADD power of +3.00 or lower. The 1.57 mid-index progressive polymer lens covers SPH of -4.00/+2.00 or less, CYL of +/- 6.00 or less, and ADD power of +3.50 or less.
The 1.53 medium index Trivex progressive lens has, like the single vision Trivex lens, the highest impact resistance of any lens we carry. Like Trivex single vision lenses, these lenses cannot be tinted. It is slightly thinner than the standard 1.50 index lens. Covers SPH of -4.00/+2.00 or less, CYL of +/-4.00 or less, and an ADD potency of +3.00 or less. It is recommended for sports and rimless eyeglasses, but unlike Trivex single vision lenses, it is not recommended for children, who are only rarely prescribed multifocal glasses.
The characteristics of the following progressive lenses, the 1.59 medium index pure polycarbonate progressive lens and the 1.61 and 1.67 high index lenses, follow the same pattern as single vision lenses. 1.59 progressive lenses, which, like Trivex lenses, cannot be tinted (although they can be ordered as glare-reducing polarized sunglasses or with “auto-tint” photochromic lenses), cover SPH from – 4.00/+2.00 or less, CYL of +/- 4.00 or less, and an ADD of +3.00 or less.
High index 1.61 progressive polymer lens covers SPH of -6.00/+3.00 or less, CYL of +/- 6.00 or less and ADD of +3.00 or less. The 1.67 high index progressive polymer lens covers SPH of -10.00/+8.00 or less, CYL of +/- 6.00 or less and an ADD of +3.00 or less. As with single vision lenses, anti-reflective coatings are recommended with high index lenses.
Now you know all the features of all prescription lenses.