Rx glasses are simply prescription glasses. “Rx” is a common abbreviation for “prescription,” borrowed from the Latin word “recipe,” and it’s used across medicine and pharmacy. When you see “Rx glasses” on a website or in an optical shop, it means eyeglasses with lenses custom-made to correct your specific vision problems, as opposed to over-the-counter reading glasses or non-prescription frames.
How Prescription Lenses Correct Your Vision
Light enters your eyes and passes through the cornea and lens, which bend it so it lands precisely on the retina at the back of your eye. If the shape of your eye, cornea, or lens is slightly off, that light doesn’t focus where it should, and things look blurry. Prescription lenses add the exact amount of extra bend your eyes can’t produce on their own, redirecting light so it hits the retina correctly.
The power of that correction is measured in units called diopters. A negative number (like -2.00) means the lens corrects for nearsightedness by spreading light outward before it enters the eye. A positive number (like +1.50) corrects for farsightedness by converging light inward. The higher the number in either direction, the stronger your prescription.
Vision Problems Rx Glasses Fix
Nearsightedness (myopia) makes distant objects look blurry while close-up things stay sharp. It typically happens because the eyeball grows slightly too long from front to back, so light focuses in front of the retina instead of directly on it.
Farsightedness (hyperopia) is the opposite. Close-up objects are hard to see clearly, usually because the eyeball is too short or the cornea isn’t curved enough, causing light to focus behind the retina.
Astigmatism blurs vision at all distances. Instead of being round, the eye is shaped more like a football, which causes light to bend unevenly and distort images. Many people have astigmatism alongside nearsightedness or farsightedness, and a single pair of Rx glasses can correct both.
Presbyopia is age-related farsightedness that develops when the lens inside your eye gradually stiffens and loses its ability to shift focus. It usually shows up after age 40 and is the reason many people who never needed glasses before start reaching for reading glasses.
Reading Your Prescription
An eyeglass prescription looks like a small table filled with abbreviations. Once you know what each column means, it’s straightforward to interpret.
- OD and OS: OD is your right eye, OS is your left eye. These come from Latin terms.
- Sphere (SPH): The main lens power needed to correct your vision, measured in diopters. A minus sign means nearsighted, a plus sign means farsighted.
- Cylinder (CYL): How much astigmatism correction you need, if any. Many people have this box empty or marked as zero.
- Axis: A number between 1 and 180 that indicates the angle of your astigmatism on the cornea. It only appears if you have a CYL value.
- Add: Additional magnifying power for reading or close work, typically included for people with presbyopia who need bifocal or progressive lenses.
Types of Prescription Lenses
Single vision lenses correct for one distance only: near, far, or intermediate. They’re the most common type, especially for people under 40 who have a straightforward prescription.
Bifocal lenses combine two prescriptions in one lens, with a visible line dividing them. The upper portion handles distance vision, and the lower portion is for reading and close-up tasks. They’re a practical choice for people who would otherwise switch between two pairs of glasses throughout the day.
Progressive lenses do the same job as bifocals but without the visible line. They transition gradually from distance correction at the top through intermediate vision in the middle to reading power at the bottom. This creates a more natural visual experience, though some people need a week or two to adjust to the way the lens zones shift as they move their eyes.
Lens Materials
The material your lenses are made from affects their weight, thickness, durability, and optical quality. Three options cover the vast majority of prescriptions.
CR-39 is a standard plastic that delivers excellent optical clarity with minimal distortion. It’s slightly heavier than the alternatives and can crack on hard impact, but it’s comfortable for everyday wear and tends to cost less.
Polycarbonate is significantly lighter than CR-39 and highly impact-resistant, making it the go-to choice for kids, athletes, and anyone who needs durable eyewear. Its optical clarity is slightly lower than CR-39, though most people won’t notice a difference.
High-index lenses are designed for stronger prescriptions. They bend light more efficiently, which means the lens itself can be thinner and lighter. If your prescription is above roughly +/- 4.00 diopters, high-index lenses prevent the thick, heavy look that standard materials can produce.
Common Lens Coatings
Anti-reflective (AR) coating is one of the most useful upgrades you can add. Standard uncoated lenses reflect about 8% of incoming light back toward your eyes, creating glare. AR coating cuts that reflection to less than 1%, which sharpens vision noticeably in low light and high-contrast situations. If you drive at night and struggle with halos around headlights, or spend long hours staring at a monitor, AR coating makes a real difference.
Blue light filtering lenses reduce the amount of high-energy visible light that reaches your retina from screens and artificial lighting. Digital eye strain, a mix of fatigue, dryness, headaches, and blurred vision, affects an estimated 50 to 90% of regular computer users. Blue light exposure in the evening can also interfere with melatonin production and disrupt sleep. If you spend three or more hours a day on digital devices, blue light filtering is worth considering.
Photochromic lenses contain light-sensitive molecules that darken when exposed to UV radiation outdoors and return to clear when you step inside. They eliminate the need to carry a separate pair of sunglasses, though they typically don’t darken inside a car because the windshield blocks most UV light.
Pupillary Distance and Lens Fit
One measurement that often catches people off guard when ordering glasses online is pupillary distance (PD), the space in millimeters between the centers of your two pupils. This number ensures the optical center of each lens lines up exactly with your line of sight. When PD is even slightly off, it can introduce distortion, eye strain, and headaches. Higher prescriptions and specialized lens designs like progressives are especially sensitive to PD accuracy.
Your eye doctor may or may not include PD on your written prescription. If it’s missing, you can ask the office to measure it, or some online retailers offer tools to measure it yourself using a mirror and a ruler.
Your Right to Your Prescription
In the United States, the Federal Trade Commission’s Eyeglass Rule requires every eye doctor to hand you a copy of your prescription immediately after your exam, whether you ask for it or not, and before offering to sell you glasses. They cannot charge an extra fee for the prescription itself, and they cannot require you to buy glasses from them in exchange for releasing it. The prescription can be provided digitally (by email, text, or online portal) if you agree to that format in writing. This means you’re free to shop for frames and lenses anywhere, from the doctor’s office to an online retailer, using the same prescription.
How Often to Update Your Prescription
Your eyes change over time, so a prescription from several years ago may no longer give you the sharpest vision. The American Academy of Ophthalmology recommends comprehensive eye exams on this schedule for adults without known eye disease or risk factors:
- Under 40: every 5 to 10 years
- 40 to 54: every 2 to 4 years
- 55 to 64: every 1 to 3 years
- 65 and older: every 1 to 2 years
If you already wear Rx glasses, you’ll likely need more frequent checks than these minimums suggest, since even small prescription shifts can cause eye strain and headaches. Most optometrists recommend updating your prescription every one to two years regardless of age.