How to Read Your Glasses Prescription

by Nov 22, 2021

When it comes to getting a new pair of glasses or contacts, the prescription can seem like a foreign language to you! What do all of these numbers mean, and why does it matter?


Testing Your Visual Acuities

One of the first things most doctors check during an eye exam is your visual acuity. This is the test where letters are shown on a chart and get progressively smaller to push the limits of your visual system.

Visual acuity charts are standardized—they are tested from a certain distance (20 feet) with specific letter sizes and fonts.

In fact, 20/20 vision stands for the ability to see, when standing at a distance of 20 feet, a size 20 letter. 20/20 vision is considered “the gold standard” of vision and is what most people aim to see through corrective lenses.

Some individuals may see better than 20/20 however, with visual acuities of 20/15 or even 20/10! In other words, the first number is the test distance (typically 20 for 20 feet) and the second number is how small of a letter you can see.

The lower the number the better your eyesight, and vice versa, the larger the lower number the worse your eyesight.


Types of Refractive Error

For those who need glasses or contact lenses to see 20/20, understanding the prescription given to you can be quite confusing. We’ll aim to break things down a bit and help you to understand what exactly your prescription means.

Before we get into specifics though, we’ll talk about some prescription basics.

The power of the eye, and power of prescriptions, uses the Diopter as a unit of measurement.

The smallest change in power the eye can optically detect is a quarter of a diopter (0.25).

The eye works as an optical system. Images are created by light coming from an object and being bent by the eye’s optical system so that the rays of light come to a point source on the retina (the backmost structure of the eye responsible for detecting light and transmitting these signals to the brain for processing).

If light does not come to a perfect point on the retina—i.e. comes to a focus in front of or behind the retina—blur is created.


There are Three Main Categories for Your Glasses Prescription —Myopia, Hyperopia, and Astigmatism



Myopia is the official name for those who are near-sighted (can see well up close but cannot see far away).

Myopia is created when the eye is too strong or too large, resulting in light being bent too much, coming to a point focus in front of the retina.

If the average eye, seeing 20/20 without the need for correction, was +60.00 Diopters, the myopic eye may be +62.00 diopters—or 2.00 diopters too strong. To correct for this over-power, we put weakening lenses (-) in front of the eye to bring it to the correct power, and thus move the point of focus from in front of the retina to directly on the retina.

For this reason, people who are myopic will have –X.XX prescriptions.



Hyperopia is the official name for those who are far sighted (can see well far away but cannot see well up close).

Hyperopia is created when the eye is too weak or too short, resulting in light being bent too little, coming to a point focus “behind” the retina.

If the average eye was +60.00 Diopter, the hyperopia eye may be +58.00 Diopters—or 2.00 diopters too weak. To correct for this under-power, we put strengthening lenses (+) in front of the eye to bring it to the correct power, and thus move the point of focus forward to land direction on the retina.

For this reason, people who are hyperopia will have +X.XX prescriptions.

What is a bit tricky about hyperopia is that our lens (located in the center of the eye) is flexible and can add power to the optical system through a process called accommodation.This is why we can see from a variety of distances—distance, intermediate, and close up. We lose the ability to accommodate around the age of 45, hence why readers become necessary.

This is important because some mild-to-moderate hyperopes may not realize they have a prescription, as their lens is adding in the extra power to make them see clearly. These individuals may note frequent eye-strain, fatigue, and headaches do to over-exertion of the accommodative muscles.



Astigmatism is something completely different from myopia and hyperopia. Astigmatism occurs when the eye is not perfectly spherical, being steeper in one meridian. Think about a basketball that is too large being myopia, and a basketball being too small being hyperopia. Astigmatism would be a football.

When light is refracted through an astigmatic eye, it is bent more in the steeper meridian than it is bent by the flatter meridian. This creates two focal points, resulting in a streaky, shadowy blur.

Astigmatism can occur in junction with myopia or hyperopia, or on its own. For this reason, astigmatic prescriptions are written as “cylinder” prescriptions (sometimes abbreviated CYL for short). The cylinder prescription is the second number on a glasses prescription.

For example, a -1.00 -1.50 X 180 prescription would indicate that the individual had 1.00 diopter of myopia and 1.50 diopters of astigmatism. The last number (X 180) denotes the axis of the astigmatism, or where exactly the steep meridian of the eye lies. Axis numbers are always written as three digits and can range from 001 to 180.

Here’s a couple more examples:

+1.00 Sph. Would indicate this individual has no astigmatism since there is only one number (+1.00). The + sign indicates hyperopia. So, this person has 1.00 diopter of hyperopia.

+2.00 -0.75 X 090 would indicate this this individual is hyperopic and has astigmatism. The 090 shows the steepest meridian is in the 090 orientation.

0.00 -2.50 X 010 would indicate this individual only has astigmatism (2.50 diopters to be exact) with the steep meridian in the 010 orientation.

-4.25 -0.50 X 075 would indicate this individual is myopic (4.25 diopters) and has astigmatism (0.50 diopters) with the steepest meridian being in the 075 orientation.

As you can see, the options are limitless!

One last thing to add into a glasses prescription is for those who have lost their accommodative abilities—called presbyopia. Presbyopia will happen to everyone eventually, typically taking effect around the age of 45.

When this happens, our lens can no longer flex to allow us to see up close and far away. To combat this, we simply need the extra power from reading glasses to see up close!

This can be accomplished using cheaters, bifocals, trifocals, or progressive lenses with a small Add power on the bottom (denoted on prescriptions as ADD).

Add powers typically range from +1.00 to +3.00. The higher the power, the closer the working distance. For those early in presbyopia, a +1.50 will typically suffice to start. For those a little further along in the process—60+, a +2.50 will usually do the trick.

Your eye doctor will help you to determine the best Add power for your individuals needs when writing your prescription. Be sure to tell your doctor where you like to hold your materials, and what exactly you wish to accomplish with your glasses (reading a book, working on cars, computer work, etc.) so that he or she can help find the perfect power for you.


Our eye doctors at Wilmington Family Eye Care in Wilmington, DE excel in prescription of glasses, contact lenses and the diagnosis of a variety of eye diseases. Call our optometrists at 302-299-1286 or schedule an eye exam appointment online if you would like to learn more or have questions about your glasses prescription. Our eye doctors, Drs. Daniel Baruffi, Joseph Goldberg, Karen Darrell and Patricia Jones provide the highest quality optometry services and eye exams in Wilmington, Delaware and its surrounding areas.

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