Eye Exam Basics: Understanding Prescription Numbers and Vision Tests

 Vision health is an essential part of overall well-being, yet many people overlook routine eye exams or feel confused when reading their prescription afterwards. If you have ever wondered what those letters, numbers, and symbols mean—or what exactly happens during an eye exam—you’re not alone. Understanding your eye exam results can help you take control of your eye health and make informed decisions about glasses, contact lenses, and treatment options.

In this guide, we’ll break down the basics: prescription numbers, vision tests, and what they mean for your eyesight.

Why Eye Exams Matter

A comprehensive eye exam does more than determine if you need glasses. It can detect early signs of eye conditions such as glaucoma, cataracts, and macular degeneration. Eye exams also check visual coordination, focusing ability, and provide insights into overall health issues like diabetes and high blood pressure.

Think of an eye exam as preventive care—not just a vision check.

Understanding Prescription Numbers

After your exam, your optometrist provides a prescription with abbreviations and values that may look complicated. Here's what they represent:

1. OD and OS

  • OD (Oculus Dexter): Right Eye

  • OS (Oculus Sinister): Left Eye

Sometimes, you'll also see OU, meaning both eyes.

2. Sphere (SPH)

This number indicates whether you are farsighted or nearsighted.

  • A minus sign (–): Nearsighted (myopia)

  • A plus sign (+): Farsighted (hyperopia)

The higher the number, the stronger your prescription.

3. Cylinder (CYL)

This measures astigmatism, a condition where the eye’s curvature isn’t perfectly round.

  • A number close to zero means little to no astigmatism.

  • A higher number indicates more correction is needed.

4. Axis

Displayed in degrees (1° to 180°), the axis describes where the astigmatism is located.

5. ADD

Found in progressive or bifocal prescriptions, ADD indicates the additional magnifying power needed for reading.

Common Vision Tests During an Eye Exam

During your appointment, your optometrist will perform several tests to evaluate different aspects of your vision and eye health.

✔ Visual Acuity Test

This is the familiar eye chart test where you read letters at different sizes. Results are expressed as 20/20, 20/40, etc.

  • 20/20 means normal vision.

  • 20/40 means you must be 20 feet away to see what someone with normal vision sees at 40 feet.

✔ Refraction Test

Using a phoropter ("better 1, or better 2?"), the optometrist determines your exact lens prescription.

✔ Eye Muscle and Alignment Test

This checks how well your eyes work together and detect issues such as lazy eye or double vision.

✔ Slit Lamp Examination

A specialized microscope helps examine the cornea, lens, retina, and optic nerve for signs of disease.

✔ Tonometry Test

This measures eye pressure to screen for glaucoma.

What Happens After the Exam?

Once your prescription and health checks are complete, you may:

  • Choose eyeglasses or contact lenses

  • Discuss corrective options such as blue-light filters or specialty lenses

  • Plan follow-up care if any eye condition is detected

Your optometrist will also recommend how often you should schedule exams—typically once every 1–2 years, or more frequently if you have underlying eye issues.

Final Thoughts

Understanding your eye exam results gives you confidence and clarity about your vision health. Your prescription isn’t just numbers—it’s a customized guide to helping you see the world clearly and comfortably. Whether you need glasses, contacts, or just routine monitoring, regular eye exams ensure your vision remains healthy for years to come.

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