Updated on 

May 4, 2022

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What is a "Normal" Eye Axis?

Eye doctors examine eyes to diagnose vision problems and refractive errors. 

One aspect that eye doctors consider is the eye axis.

The axis is the lens meridian that does not contain cylinder power. It's defined by a number between 1 and 180 degrees.

  • The number 90 represents the eye’s vertical meridian
  • The number 180 represents the horizontal meridian
How to tell eyeglass prescription

The axis does not indicate the strength of an eyeglass prescription. Instead, these numbers reflect the location of any astigmatism present. 

This will be helpful for opticians when an eyeglass prescription has cylinder power (CYL value). The axis value directs where to place the power in the lenses. This allows people with astigmatism to see better. 

In eyeglass prescriptions, optometrists will note both the cylinder power and axis. The cylinder power is the strength of astigmatism correction needed. If the axis is 180 degrees, you may see it noted as x180.

Those with a “normal” eye axis will not see these values in their eyeglass prescriptions. This is because these people don't have astigmatism. 

People with a normal eye axis have strong and clear vision. This is because the light falls at the right place on the retina.

Summary

The numbers linked with the eye axis determine the presence, degree, and location of astigmatism. A normal eye axis means having a strong and clear vision.

What Do Eyeglass Prescriptions Mean?

The eye axis is not the only value that eyeglass prescriptions may carry. During vision exams, eye care specialists will note other values to determine the degree of vision problems present. They use this to correct your vision and improve your visual acuity.

The following abbreviations and names are all elements that can appear in an eyeglass prescription:

OD and OS

These abbreviations stand for the Latin words oculus dexter and oculus sinister. The former refers to the right eye. The latter refers to the left eye. 

SPH

This abbreviation stands for sphere. It represents the lens power needed to correct vision problems.

If there is a minus sign (-) beside a number, the person has some degree of myopia (nearsightedness).

If there is a positive sign (+), the person has some degree of hyperopia (farsightedness).

Add

An optician may write a value here to describe an additional lens power needed for reading. This value can serve for multifocal lenses. For example, bifocals or trifocals. 

Prism

People with double vision problems will need a specific type of correction manufactured into the lens. 

Double vision problems refer to when someone sees two different images of the same object.

Prescriptions for eyeglasses and contact lenses will never be the same. The information provided in an eyeglass prescription will not be enough for a contact lens prescription and proper fitting.

Farsighted Prescriptions 

In farsighted prescriptions, eye care specialists write a plus sign (+) and a number to show how much lens power is necessary for vision correction. Lens power is represented in diopters. 

The further away the number is from zero (assuming that no correction is needed), the stronger the prescription is. 

To determine the degree of farsightedness (distance vision), eye care specialists may use various tools.

These instruments include:

  • Retinoscope. This instrument shines special light into the eye
  • Autorefractor. This instrument takes the place of a retinoscope
  • Phoropter. This tool measures for a person's refractive error

Nearsighted Prescriptions 

In nearsighted prescriptions, eye care specialists write a minus sign (-) and a number to show how much lens power is necessary for vision correction. Lens power is represented in diopters. 

The further away the number is from zero (assuming that no correction is needed), the stronger the prescription is. 

Around 42 percent of people in the United States have myopia (nearsightedness).4 These people may face an increased risk of developing glaucoma and cataracts. 

Bifocal Prescriptions (Multifocal) 

When eyeglass prescriptions have an add value, people may sport multifocal eyewear. For example, bifocals or trifocals.

This option may be appealing to those who do not want to wear separate glasses for reading and other activities.

For example, with bifocals, wearers can use the lower half of the lenses with the added power to read or more. 

Astigmatism Prescriptions 

Eye care doctors use two specific values to describe the degree and location of astigmatism. Those two values are cylinder and axis. 

While many people have somewhere between 0.5 and 0.75 diopters of astigmatism, the perfect eye with no astigmatism has 0 diopters.5

Those with 1.5 or more diopters will generally need contacts or eyeglasses for improved vision. 

Prism Correction Prescriptions

Prism correction prescriptions are for people who have diplopia (double vision) due to eye misalignment.

Different factors may contribute to eye misalignment, including:

  • Eye muscle problems. For example, strabismus (crossed eyes)
  • Neurological conditions. For example, stroke or migraine
  • Nerve-related issues. For example, multiple sclerosis or diabetes mellitus

To measure and prescribe a prism, optometrists will use prism diopters. For example, 0.5 PD or 1.5PD, and so on.

The prism may be positioned vertically, horizontally, or diagonally in one or both eyeglass lenses. 

In some cases, optometrists may even place a temporary Fresnel prism to give an idea of how an actual prism would work. A Fresnel prism is a thin vinyl sticker.

The Fresnel prism could be used when this type of eye prescription is about to change shortly.

Summary

There are various types of eyeglass prescriptions. Farsighted prescriptions have a plus (+) sign. Nearsighted prescriptions have a minus (-) sign. Bifocal prescriptions have an Add value. Other types of prescriptions include astigmatism and prism correction prescriptions.

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6 Cited Research Articles
  1. Gilbert , Michael L. “What Are ‘Normal’ Results of an Eye Exam? Other than 20/20, What Are Normal Axis and Sphere Numbers?” American Academy of Ophthalmology, 8 Mar. 2018
  2. Porter, Daniel. “How to Read an Eyeglasses Prescription.” American Academy of Ophthalmology, 15 July 2020
  3. Porter, Daniel. “What Is Prism Correction in Eyeglasses?” American Academy of Ophthalmology, 5 Mar. 2019
  4. Why Has Myopia Become More Common?, Pacific University Oregon, January 2019
  5. What Do Astigmatism Measurements Mean?, American Academy of Ophthalmology, April 2021
  6. Navarro, Rafael. “The Optical Design of the Human Eye: a Critical Review.” Journal of Optometry vol. 2,1 : 3–18
Melody Huang is an optometrist and freelance health writer. Through her writing, Dr. Huang enjoys educating patients on how to lead healthier and happier lives. She also has an interest in Eastern medicine practices and learning about integrative medicine. When she’s not working, Dr. Huang loves reviewing new skin care products, trying interesting food recipes, or hanging with her adopted cats.
Anthony Armenta earned his B.A. in International Relations from the University of California, Irvine. After graduation, he decided to live abroad in Spain. Currently, he has spent the past 5 years working as a freelance health content writer and medical editor for different public hospitals in central Barcelona. He has covered different medical specialties from infectious diseases and pneumology to breast cancer and plastic surgery. His commitment to writing fact-driven, health-related content stems from the belief that such type of information can empower all individuals to take action and improve their health today.
https://www.visioncenter.org/author/anthony/
Author: Anthony Armenta  | UPDATED May 4, 2022
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Medical reviewer: Dr. Melody Huang, O.D.
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Dr. Melody Huang, O.D.
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The information provided on VisionCenter.org should not be used in place of actual information provided by a doctor or a specialist.

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