Eyeglass Prescriptions Explained

eye 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 statistic is significant, it is essential to understand that what drives adults to wear eyewear is the prescription. Unfortunately, for many, they don’t know what their prescription means, so we at SafeVision are going to break it down for you.

How The Eye Works

When we open our eyes and allow the light to enter, many parts work together to help us see the world around us. Scattered light will come together and touch the top layer first, called the cornea, where it passes through to the pupil. The pupil is the centered second layer that acts as a tunnel to allow the light into the back of our eyes. Once it passes through, it reaches the lens.

The lens works with the cornea to fine-tune our vision before sending it to the complex layer of cells that react to light known as the retina. But once inside, the images shrink and flip to match the shape of the retina. The light is then relayed from the retina to the brain and translated into the images we see before us.

Vision Issues

In a perfect world, we would all have a 20/20 vision. But unfortunately, that is not the case for most of the population. Some of the most common problems in vision are:


  • Myopia– Also known as nearsightedness, which means you can see things 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. It occurs when the shape of your eye causes light rays to bend (refract) incorrectly, focusing images in front of your retina instead of on your retina.


  • Hyperopia– Farsightedness is a common vision condition in which you can see distant objects clearly, but objects nearby may be 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 usually is present at birth and tends to run in families.


  • Astigmatism-Understanding astigmatism and what is happening with the eye is the first step. 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.

For those with astigmatism, the cornea is shaped like a football rather than a basketball. This curvature creates a slightly distorted image. Astigmatism ranges in severity, with some people seeing images that are blurry or stretched out. Both children and adults are at risk of developing astigmatism. In some cases, people do not even realize they have astigmatism until they go in for a comprehensive eye exam.

  • Presbyopia– is one of the most common age-related eye conditions. This eye disease results in blurred near vision and usually starts at around the age of 40. 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.

The word “presbyopia” means “old eye” in Greek, which is representative of the eye condition and what is happening. It is believed that presbyopia is caused by changes in the lens inside the eye. As we get older, the lens gets harder and less elastic. When this occurs, it becomes much more difficult for the eye to focus on close objects, whether you are trying to read a text message or a book.

How A Prescription Eyeglass Lens Works

To fully understand how a prescription works, you need to know how a lens works first. A lens is two rounded prisms joined together where one part is usually thicker. When the light enters the lens, it is bent/refracted toward the thicker end of the lens. But, what type of lens you have determines how the light is bent.

  • Spherical Lens: This lens has a curve on the front of the lens and a curve on the back. If the thickest part of the lens is on the outer edge, the lens corrects for nearsightedness. If the thickest part of the lens is in the center of the lens, the lens corrects for farsightedness.
  • Cylindrical Lens: Corrects for astigmatism. It has a curve on the front of the lens and two curves on the back side of the lens.

Lens strength is also an important part of a prescription, and doctors use diopters to express how strong a lens is. Higher diopters indicated stronger lenses and how much light is bent.

  • 1 diopter = 1 m
  • 2 diopters = 0.5 m
  • 3 diopters = 0.33 meters, etc.

Another important part of a prescription is the lens type. There will either be a plus (+) or minus (-) sign before the diopter telling you if your lens is convex (+) or concave (-).

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. Typically, you will see many abbreviations and words describing all the data; we will break some of those down for you.

  • OD = Comes from the Latin term oculus dexter, which means right eye.
  • OS = Comes from the Latin term oculus sinister, which means left eye.

You will see these often on eyewear prescription charts. Here is the shorthand of an optometrist broken down:

OD +2.50 -1.50 X 123 plus +2.00 add

  • OD = right eye
  • 2.50 = spherical base strength and +/- type [+2.5D]
  • -1.50 = cylinder strength and type [-1.50D cylinder]
  • X 123 = cylinder axis orientation (x means “at”) [123 degrees]
  • +2.00 = strength of multifocal segment] – this section is added for bifocals, trifocals or progressive lenses

How To Incorporate Your Prescription Into Your Eyewear

It is no secret that wearing the correct prescription is essential for your vision. That’s why here at SafeVision, all of our safety eyewear lenses are customizable to your exact prescription, so you don’t have to worry about your vision on the job.

USA and Canada regulations require employers to ensure that all employees have appropriate eye or face protection if exposed to potential hazards. General dress eyewear (streetwear or non-safety frames) does not meet the standards set forth by ANSI / CSA Standards for Safety Prescription 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.

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.

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