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How To Understand Diopter in Your Eye Prescription

Have you ever glanced at your eye prescription and wondered how they arrived at all those numbers? You know you go to the optometrist, look through a few lenses at an eye chart, and voilà, you have a prescription. However, the diopter is how they arrived at that prescription.

You're not alone if you've been wearing glasses for years and have never heard of a diopter. Eye prescriptions have numbers and terms corresponding to your vision needs, and we'll help you understand yours. We're diving into the diopter, and you'll feel like you're on your way to being an optometrist yourself.

What Is Diopter?

Simply put, a diopter is a unit of measurement that correlates to a lens's optical power. But what does that really mean when it comes to your eyes? Think of your eyes like a camera, but the focus is broken. The camera needs help to refocus on images.

The diopter is able to measure refractive errors and prescribe lenses to correct them. The numbers on your prescription indicate the strength of the lenses required to bring your vision back into focus.

These diopter numbers can also be positive or negative. A plus sign in front of the numbers means you're farsighted, while a negative diopter value means you're nearsighted. For example, if your prescription reads -3.00, your glasses need a three-diopter correction to correct nearsightedness. If your prescription is +4.50, you need a 4.5 diopter correction to fix farsightedness.

The higher the number, the more assistance your eyes need for clear vision. If you struggle to see, it may be time to see an eye doctor to correct your vision. They can determine the strength of the lenses you need. Remember, your eyes get weaker over time, so it's essential to keep up with regular eye exams for an up-to-date prescription.

How To Understand Your Eye Prescription

Let's get down to the nitty-gritty of your eye prescription. There are a few key components essential for understanding your eyes.

Here's a breakdown:

  • Sphere (Sph): The sphere is abbreviated "Sph" on your prescription. It specifies the lens power needed to correct your refractive error.
  • Cylinder (Cyl): The cylinder, or "Cyl" on your prescription, represents the degree of astigmatism in your eyes. Astigmatism is an irregular curve in your eye on either the lens or the cornea. If you have astigmatism, you'll require additional lens power.
  • Axis: The axis goes hand-in-hand with the cylinder value. It specifies where astigmatism appears in your eye. The axis will be a number between 0 and 180. You can have horizontal or vertical astigmatism.
  • Addition (Add): This section is for those with presbyopia, a common age-related condition affecting near vision. The number here gives the lens power needed for seeing close-up objects.
  • Base Curve (BC) and Diameter (DIA): People who wear contact lenses may notice their prescription includes specifications for the lenses' base curve (BC) and diameter (DIA). These measurements are essential to ensure a proper fit and optimal vision correction. No one wants contacts that slide around and aren't comfortable.

What Are Common Eye Prescription Terms?

Your vision becomes blurry when your eye prevents light from focusing correctly on your retina. There are four common types of refractive errors. We've already briefly touched on three of them, but let's examine them more closely.


Nearsightedness, or myopia, means people can only see objects close to them. When a nearsighted person looks farther away, the world becomes blurry.

People with myopia typically have elongated eye shapes, with too much space between the cornea and the retina. It can also occur in people with curved corneas. The distance causes light rays to fall in front of the retina instead of on it. Corrective lenses will correct the bend and bring objects far away into focus.


If you've ever seen someone at a restaurant hold their menu far away to try to read it, you've seen someone struggle with farsightedness.

This refractive error causes blurriness in objects close by but not far away. One reason for this vision issue is that the distance between the cornea and the retina is too short. Farsightedness, or hyperopia, can also occur because the cornea isn't curved enough.


Astigmatism results from an irregular curve on the lens or the cornea. The curve causes light to bend as it enters your eye, and your vision is blurry, both near and far. Astigmatism can also distort images.


Presbyopia is when the eyes slowly stop being able to see objects close up. It occurs in adults 45 and older and is a normal part of aging. This process occurs as the lens in your eye ages and becomes rigid, making it harder to focus light onto the retina.

How To Find Prescription Glasses

Thankfully, you don't have to understand every part of your diopter measurement to get the right prescription glasses — your eyewear provider can read your prescription and provide you with the right lenses. That said, it's still a good idea to understand your unique needs.

Once you have your eye prescription, you can order new glasses. Look for a brand with frames that fit your needs and style. Pair Eyewear has several high-quality options (but we’re not biased or anything).

Our lenses made in our state-of-the-art lens lab can correct nearsighted, farsighted, and those with astigmatism. Plus, they’re treated with anti-reflective and anti-scratch treatments to make sure you can always see clearly.

You can even add Sun Tops to your order, which are tinted lenses in a frame that snaps right onto your prescription glasses. What does that mean? No more needing to order two separate pairs of eyewear just to be able to see outside.

Stay in Focus

We hope you feel a little more informed about your prescription. Understanding your specific needs can help you make more informed decisions about your vision care. Continue to go in for regular eye exams in case your diopter reading changes. Pair Eyewear can provide you with great glasses with any prescription.


How to Read an Eye Prescription: What the Numbers Mean | Healthline

Refractive Errors | National Eye Institute

Presbyopia - Symptoms and causes | Mayo Clinic

Cornea: What It Is, Common Disorders & Preventing Injury | Cleveland Clinic