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How to Read an Eye Prescription – Complete Guide for Beginners

Jul 26, 2025

1. Understanding the Structure of an Eye Prescription

Looking at your eye prescription can feel like trying to decipher a secret code. But once you understand how to read an eye prescription, everything becomes much clearer—literally and figuratively. Whether you're getting glasses for the first time or updating your lenses, knowing what those numbers mean can help you make better choices for your vision health.

An eye prescription typically consists of abbreviations, numerical values, and sometimes specialized notations. These are tailored to correct specific visual issues such as nearsightedness, farsightedness, astigmatism, or age-related focus difficulties. Understanding this layout is key to avoiding confusion when purchasing lenses or comparing treatment options.

2. What Each Abbreviation Means

2.1 OD and OS – Which Eye Is Which?

OD stands for "oculus dexter," meaning your right eye, and OS is "oculus sinister," for your left eye. Occasionally, you’ll also see OU, meaning both eyes. These Latin terms are standard in optical prescriptions globally.

2.2 SPH – Spherical Correction

This number corrects nearsightedness (negative value) or farsightedness (positive value). For example, -2.00 SPH means you're moderately nearsighted, needing help to see far-away objects.

2.3 CYL – Cylinder

This corrects astigmatism and will only appear if you have it. Like SPH, it may be positive or negative. The higher the number, the more correction is needed for irregular curvature of the cornea.

2.4 AXIS – Direction of Astigmatism

The axis, measured in degrees (1 to 180), shows the orientation of astigmatism. Think of it as the angle at which the lens should be positioned to properly correct your vision.

2.5 ADD – Reading Correction

Often seen in prescriptions for bifocal or progressive lenses, ADD refers to the added magnification needed for near vision. This is common for individuals over 40 dealing with presbyopia.

2.6 PD – Pupillary Distance

This measurement (usually in mm) indicates the space between your pupils and is crucial for correctly aligning lenses in your frames. A wrong PD can cause eyestrain or headaches.

3. Real-World Example of an Eye Prescription

Let’s look at a sample prescription and break it down: 

OD: -1.75 SPH / -0.75 CYL x 180
OS: -2.00 SPH / -1.00 CYL x 170
ADD: +1.50
PD: 63mm

Here’s what this means:

  • The person is nearsighted in both eyes.
  • They also have astigmatism, with different degrees and axis orientations in each eye.
  • The ADD value suggests they need additional magnification for reading.
  • A PD of 63mm helps the lab center the lenses appropriately within the frames.

4. Common Questions First-Time Patients Ask

4.1 “Why Do My Eyes Have Different Numbers?”

It’s completely normal for each eye to have different correction needs. This is called anisometropia. A proper prescription ensures balance and comfort between both eyes.

4.2 “Can I Use My Prescription for Both Glasses and Contacts?”

Not exactly. A contact lens prescription includes additional measurements like base curve and diameter. So, you’ll need a separate exam if switching to contacts.

4.3 “Is My Vision Getting Worse?”

Slight changes in SPH or CYL over time are common. Your optometrist will compare results year to year. If changes are rapid or significant, further tests may be recommended.

5. How Eye Docs Helps You Navigate Prescriptions

Decoding your eye prescription shouldn’t be intimidating. At Eye Docs, we offer detailed vision support, trusted optometrists, and a wide selection of lenses and frames—all tailored to your specific needs.

With personalized guides, online tools to input your prescription, and customer support to double-check your order, Eye Docs ensures you never have to second-guess your eyewear choices. Whether you're a first-time glasses wearer or just updating your look, we help turn complicated numbers into clear, confident vision.