Optometrist & Eyewear

Understanding Your Eyeglass Prescription

We know you trust your optometrist to complete an accurate eye exam and ensure that you receive the right prescription to suit your vision needs.

But when they hand you your prescription you may be wondering: What does all of this mean?

While it may seem like a jumble of letters and numbers, to an optometrist it provides all the information they need to provide you with the right eyeglasses to correct your vision.

However, it’s understandable that you want to understand your prescription as well and, to be honest, it’s not as complicated as it seems.

In this article, we are going to use a sample prescription to explain what the prescription numbers mean and how they are used to provide you with optimal vision:

Here’s an Example:

Below is an example prescription that likely looks familiar to you! We’re going to use this to explain what your prescription numbers mean.

example eyeglass prescription from optometrist

Rx: OD and OS

Your actual eyeglass prescription (Rx) is based on two abbreviations: OD and OS.

These abbreviations are based on Latin terms with Oculus Dexter (OD) indicating the right eye and Ocular Sinister (OS) indicating the left eye.

You may also see the abbreviation OU (Oculus Uterque) which refers to both eyes. More often than not, however, you will have two separate prescriptions for each eye.

Spherical Correction (SPH)

To the right of the abbreviations OD and OS, you will find numbers. This is the spherical correction (SPH) and indicates how powerful each lens has to be to correct your vision.

If the number is noted with a “-”, you have nearsightedness. Farsightedness is noted with a “+”.

The SPH is measured in dioptres which is the unit used to measure the focusing power of the lens based on what your eyes require.

When it comes to the actual numbers, the further away from zero, the stronger your prescription has to be to correct your vision. 

If we refer to the example above, the prescription indicates that the right eye requires an SPH of -2.00 while the left eye requires -2.75. This means that the patient is nearsighted with 2 diopters of nearsightedness in the right eye and 2 and ¾ in the left.

Cylindrical Correction (CYL)

The next number in the prescription is the cylindrical correction (CYL) and is only filled in if you have astigmatism.

The CYL measurement indicates to what degree you have astigmatism (i.e. how flat or irregular the shape of your cornea is). The more oval-shaped your eyes are, the more astigmatism you have.

Astigmatism is also measured in diopters. If there is no number is in this column, you either don’t have astigmatism or not enough to require additional vision correction.

Otherwise, you’ll notice a “+” or “-” here as well which also indicates nearsightedness or farsightedness.

In our above example, the patient requires -1.00 correction in the right eye and –0.75 correct in the left eye for nearsighted astigmatism.


The last number you will see on your eyeglass prescription is the Axis. This number indicates an angle from 0 to 180 degrees to where on the cornea the astigmatism is located.


This abbreviation simply indicates that something has to be “added” to your prescription such as additional power to provide clear vision when reading or doing other activities close to your eyes.

This is typically used when it comes to prescribing bifocals and reading glasses.

Pupillary Distance (PD)

Lastly, you may see the abbreviation PD on your prescription. This represents pupillary distance, or the distance between your pupils.

This is used to ensure that your eyeglasses are comfortable and optically efficient by aligning your glasses directly in front of the center of your pupil.

Other Abbreviations

Here are some other abbreviations you may see on your eyeglass prescription that we did not use in our example:

  • SVD: Single Vision Distance (used for distance vision correction only)
  • SVN: Single Vision Near (reading glasses only)
  • Prism: This measurement is not common and is used for patients who are cross-eyed or have other eye muscle or focusing disorders.

Understanding Your Prescription Number

When it comes to understanding your eyeglass prescription, you will notice a combination of letters and numbers that describe both the shape of your eye and what correction you will need in your glasses.

If ever you are confused by your prescription number, feel free to reach out to an expert optometrist!

Our team at The Eye Clinic is here to answer all of your questions. Feel free to get in touch with us for more information about eyewear prescriptions.

We also offer eye exams and LASIK surgery!

Dr. Harpal Grewal

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