Eyeglass Prescriptions Explained

Did you know that about 75% of adults use some type of vision correction?

According to the Vision Council, 64% wear eyeglasses, and about 11% wear contact lenses—either with or without glasses. While this number is significant, it’s important to understand that what drives the need for eyewear is the optical prescription. However, many people don’t know what their prescription actually means. At SafeVision, we’re here to break it down.

How The Eye Works

When we open our eyes and allow light to enter, several components work together to create visual images. Scattered light first touches the cornea, the clear outer layer. From there, it passes through the pupil, the dark circular opening in the center of the eye, which acts as a tunnel to guide light further inside.

Next, light reaches the lens, which works with the cornea to fine-tune focus before sending visual information to the retina, a complex layer of light-sensitive cells. Once inside, the image is flipped and shrunk to fit the shape of the retina. The optic nerve then transmits this information to the brain, where it is interpreted into what we see.

Common Vision Issues

In a perfect world, everyone would have 20/20 vision—but most people require some form of correction. Here are the most common problems in vision:

Myopia (Nearsightedness)

You can see objects up close but not far away. Nearsightedness is a common vision condition in which you can see objects near to you clearly, but objects farther away are blurry. This occurs when the shape of the eye causes light to focus in front of the retina instead of on it. Myopia affects a growing number of individuals, particularly children and teens.

Hyperopia (Farsightedness)

You can see distant objects clearly, but near objects appear blurry. The degree of your farsightedness influences your focusing ability. People with severe farsightedness may be able to clearly see only objects a great distance away, while those with mild farsightedness may be able to clearly see objects that are closer. Farsightedness typically results from a shorter-than-normal eyeball or a flatter cornea. It is often hereditary and usually present from birth.

Astigmatism

Refraction is the bending of light as it passes through one object to another, and then into the eye. We are able to see clearly when the light rays are refracted as they pass through the cornea and lens. The retina then focuses the light and converts it into messages sent through the optic nerve straight to the brain. At this point, the brain interprets these messages, turning them into the images we see.

Astigmatism is caused by an irregularly shaped cornea—more like a football than a basketball. This disrupts how light is focused on the retina, leading to distorted or blurry vision at all distances. It can occur alongside myopia or hyperopia and is common in both children and adults. In some cases, people do not even realize they have astigmatism until they go in for a comprehensive eye exam.

Presbyopia

Presbyopia is an age-related condition that typically starts around age 40. The word “presbyopia” means “old eye” in Greek, which is representative of the eye condition and what is happening. It results in difficulty focusing on close objects due to the lens becoming less flexible over time. A common early symptom is the need to hold reading materials farther away. 

Anyone, including those who have never had vision troubles before, are at risk of developing presbyopia. Even individuals who are already nearsighted may notice their near vision is starting to blur when wearing contact lenses or their usual eyeglasses. Some of the first things you will notice if presbyopia is developing is that you have to squint or hold reading materials at arm’s length in order to help your eyes focus. Unfortunately, there is nothing you can do to lower your risk of developing presbyopia.

How A Prescription Eyeglass Lens Works

To understand how an eyeglass prescription works, it helps to understand how corrective lenses function. A lens is made of two joined prisms that bend light toward the thickest edge. The shape of the lens determines how the light is refracted and whether it corrects for near or farsightedness.

Spherical lens

This lens has a curve on the front of the lens and a curve on the back. If thickest at the edge, it corrects for nearsightedness; if thickest in the center, it corrects for farsightedness.

Cylindrical lens

This lens is used to correct astigmatism. It has a curve on the front of the lens and two curves on the back side of the lens.

Diopters & Lens Strength

Lens strength is measured in diopters, which indicate how much the lens bends light. Higher diopters indicated stronger lenses and how much light is bent.

• 1 diopter = focuses light at 1 meter

• 2 diopters = 0.5 meters
• 3 diopters = 0.33 meters, etc.

Another important part of a prescription is the lens type. There will either be:

• A plus (+) sign: Indicates a convex lens for farsightedness or presbyopia.

• A minus (-) sign: Indicates a concave lens for nearsightedness.

How To Read A Prescription

Now that we know the basics of how the eye works and the types of lenses available, we can get into how to read a prescription. When you look at your prescription, it describes precisely how your lens should be cut. Prescriptions are written using abbreviations and numbers that define how lenses should be shaped. 

Here’s a breakdown:

• OD: Oculus dexter – right eye
• OS: Oculus sinister – left eye
• SPH (Sphere): The base strength of the lens, correcting nearsightedness or farsightedness
• CYL (Cylinder): The amount of correction needed for astigmatism
• AXIS: The orientation of the cylinder correction in degrees (from 1 to 180)
• ADD: Additional power for bifocal or progressive lenses, usually for presbyopia

Here is the shorthand of an optometrist broken down: OD +2.50 -1.50 x 123 +2.00 add

This means the right eye requires +2.50 spherical correction. Includes -1.50 cylindrical correction for astigmatism, positioned at a 123-degree axis, +2.00 added for reading or near-vision support.

How To Incorporate Your Prescription Into Your Eyewear

Wearing the correct prescription is crucial—not just for comfort, but for safety and performance in hazardous work environments. At SafeVision, all prescription lenses are custom-cut to meet your exact prescription needs.

General-use glasses or “streetwear” frames do not meet the standards for workplace protection. In the U.S. and Canada, employers are required by ANSI Z87.1 and CSA Z94.3 standards to ensure employees exposed to hazards have proper prescription safety eyewear. The ANSI Z87.1 (USA) and CSA Z94.3 (Canada) standards specify that safety prescription eyewear must meet stringent design, structural, and lens retention requirements. SafeVision prescription safety eyewear has been rigorously tested to meet the requirements of ANSI Z87.1 and CSA Z94.3.

Understanding your eyeglass prescription can help you make better decisions about vision correction and workplace safety. Whether you’re addressing nearsightedness or choosing a lens that supports your day-to-day tasks, accurate prescriptions and quality lens materials are essential. When you work with us as SafeVision, we’ll ensure your vision is protected, and your prescription is up to date to keep everyone safe.

Frequently Asked Questions