To get a glasses prescription, you need an eye exam from an optometrist or ophthalmologist. The exam typically takes 20 to 30 minutes, and at the end, your doctor is required to give you a copy of your prescription, which you can use to buy glasses anywhere you choose.
What Happens During the Eye Exam
The exam has two main parts. First, you’ll read letters on an eye chart at various distances, covering one eye at a time. This measures your visual acuity and determines whether you have 20/20 vision or need correction.
Next comes the refraction test, which is the part that actually generates your prescription. You’ll look through a device called a phoroptor, a large instrument with dozens of different lenses built in. Your doctor flips between lenses and asks “which is clearer, one or two?” over and over, narrowing down the exact lens power that gives you the sharpest vision. This back-and-forth might feel imprecise, but small differences between your answers help your doctor zero in on the right correction for each eye.
Beyond measuring your prescription, a comprehensive exam also checks for eye diseases like glaucoma, cataracts, and retinal problems. Many of these conditions have no early symptoms, so the exam serves a dual purpose even if you’re only there for glasses.
Optometrist or Ophthalmologist
Either one can give you a glasses prescription. Optometrists function like primary care providers for your eyes. They handle routine exams, diagnose common vision problems like nearsightedness and astigmatism, and complete four years of optometry school plus a residency. For most people getting a straightforward glasses prescription, an optometrist is the right choice.
Ophthalmologists are medical doctors who complete medical school, an internship, and a three-year residency. They can do everything an optometrist does, plus perform surgery and manage complex conditions like retinal detachment. If your optometrist spots something concerning during your exam, they’ll refer you to an ophthalmologist for further treatment.
Your Right to Your Prescription
Federal law requires your eye doctor to give you a copy of your prescription after the exam, whether you ask for it or not. This is enforced by the Federal Trade Commission under the Eyeglass Rule. You’re free to take that prescription to any retailer, online or in-store, and you don’t have to buy glasses from the office where you had your exam.
Most states require the prescription to include your name, the exam date, an expiration date, and the prescriber’s signature and contact information. Expiration timelines vary by state but are commonly one to two years. After that, you’ll need a new exam to get an updated prescription.
How to Read Your Prescription
A glasses prescription looks confusing at first, but each abbreviation represents a specific measurement. OD (or RE) refers to your right eye. OS (or LE) refers to your left eye. OU means both eyes.
The most important number is SPH, short for sphere. This is the main power of your lens, measured in units called diopters. A negative number means you’re nearsighted. A positive number means you’re farsighted. The higher the number in either direction, the stronger the correction you need.
If you have astigmatism, your prescription will also include CYL (cylinder) and Axis values. Astigmatism means your cornea is slightly oval rather than perfectly round, so light bends unevenly. CYL measures how much extra correction that irregular shape requires, and Axis pinpoints where on your cornea the irregularity sits, on a scale of 0 to 180 degrees. These two numbers always appear together.
A few less common entries you might see:
- Add: An additional power added to the bottom portion of the lens for reading. This is typical in bifocal or progressive prescriptions, usually for people over 40.
- Prism: A correction that redirects light to compensate for eye alignment problems. It includes a direction notation (up, down, in, or out) to tell the lab how to position the lens.
- PD (pupillary distance): The distance between your pupils in millimeters. This tells the lens maker where to place the optical center of each lens so it lines up with your eye.
Getting Your Pupillary Distance
PD is essential for ordering glasses, but some doctors don’t include it on your written prescription. If yours is missing, you have a few options.
The most accurate method is having it measured in-office with a pupillometer, a small device your eye care provider holds up to your face. You can ask for this measurement at your exam. Many optical shops will also measure it for free, even if you’re not buying from them.
You can also measure it yourself with a millimeter ruler. Close your left eye, align the zero mark over the center of your right pupil, then open your left eye and read the number that lines up with your left pupil. The average adult PD falls between 58 and 65 millimeters. Do it a few times to make sure you’re getting a consistent number.
Several online retailers and eyewear companies offer web-based PD measurement tools that use your phone or computer camera. You typically hold the screen at arm’s length, face a bright light source, and hold still for a few seconds while the software maps your pupil positions. These tools work well for most people but may struggle with very dark irises or poor lighting.
Online Vision Tests
A handful of companies now offer remote vision tests that can generate or renew a glasses prescription without an in-person visit. These are convenient, but they come with significant limitations.
The American Academy of Ophthalmology recommends online vision testing only for healthy adults between 18 and 39 who already have a mild or moderate prescription and no symptoms of eye disease. If you have diabetes, a family history of glaucoma, a high-power prescription, or haven’t had a comprehensive eye exam recently, these tests aren’t appropriate for you.
The core issue is that online tests can measure lens power but cannot detect eye diseases. Many serious conditions, including glaucoma and early retinal damage, develop without noticeable symptoms. A comprehensive in-person exam catches these problems. An online refraction test does not. For people who qualify, online tests can be a reasonable option between full exams, but they don’t replace the exam itself.
Where to Use Your Prescription
Once you have your prescription and PD, you can buy glasses from your eye doctor’s office, a retail optical chain, a warehouse club, or any of dozens of online retailers. Prices vary dramatically. The same prescription that costs $400 at one shop might run $50 online, depending on frame choice, lens coatings, and whether you need progressives or single-vision lenses.
When ordering online, you’ll enter each value from your prescription into the retailer’s form: SPH, CYL, Axis, and Add if applicable. Double-check every number and its positive or negative sign, since even a small error can result in lenses that cause headaches or blurred vision. If your prescription includes prism, ordering in person is generally a better idea, since prism lenses require more precise fitting.