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.
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.
Next, we’ll look at the specific single-vision lenses, with the prescriptions they accommodate.