What Is a "Normal" Eye Axis?

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What Is a "Normal" Eye Axis?

An eye doctor will examine an individual’s eyes to determine any vision problems or refractive errors. 

One aspect that eye doctors will consider is the eye axis. The axis is the lens meridian that does not contain cylinder power, defined by a number between 1 and 180 degrees. The eye doctor will determine the axis during a refraction (vision test). The number 90 represents the eye’s vertical meridian, whereas 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, so individuals with astigmatism can see better. 

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


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Individuals with a “normal” eye axis will not see these values in their eyeglass prescriptions because they do not have astigmatism. 

What Do Eyeglass Prescriptions Mean?

The eye axis, however, 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. 

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, while the latter refers to the left eye. 
  • SPH. This abbreviation stands for sphere and represents the lens power needed to correct vision problems. If there is a minus sign (-) beside a number, the individual has some degree of myopia (nearsightedness). On the other hand, if there is a positive sign (+), the individual 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, such as bifocals or trifocals. 
  • Prism. Individuals with double vision problems (two different images of the same object) will need a specific type of correction manufactured into the lens. 

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 will write a plus sign (+) and a number to indicate 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 a retinoscope (that shines special light into the eye), autorefractor (which takes the place of a retinoscope), and phoropter (a measurement tool for an individual’s refractive error). 

Nearsighted Prescriptions 

In nearsighted prescriptions, eye care specialists will write a minus sign (-) and a number to indicate 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. 

Nearly 50% of individuals in the United States have myopia (nearsightedness). These individuals may face an increased risk of developing glaucoma and cataracts. 

Bifocal Prescriptions (Multifocal) 

When eyeglass prescriptions have an Add value, individuals may sport multifocal eyewear, such as bifocals or trifocals. This option may be appealing to those individuals who do not want to wear separate glasses for reading, etc. 

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

Astigmatism Prescriptions 

As mentioned before, eye care doctors will use two specific values to describe the degree and location of astigmatism. Those two values are cylinder and axis. 

While many individuals have somewhere between 0.5 and 0.75 diopters of astigmatism, the perfect eye with no astigmatism (ideal curvature of the cornea) will have 0 diopters. Individuals with 1.5 or more diopters will generally need contacts or eyeglasses for improved vision. 

Prism Correction Prescriptions

Prism correction prescriptions are for individuals who have diplopia (double vision) due to eye misalignment. Different factors may contribute to eye misalignment, including:

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

To measure and prescribe a prism, optometrists will use prism diopters, such as 0.5 PD or 1.5PD, etc. 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 (a thin vinyl sticker) to give an idea of how an actual prism would work. The Fresnel prism could be used when this type of eye prescription is about to change shortly.

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Author: Anthony Armenta | UPDATED November 11, 2020
Medical reviewer: MELODY HUANG, O.D. 
Resources

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, www.aao.org/eye-health/ask-ophthalmologist-q/normal-eye-exam.

Porter, Daniel. “How to Read an Eyeglasses Prescription.” American Academy of Ophthalmology, 15 July 2020, www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription.

Porter, Daniel. “What Is Prism Correction in Eyeglasses?” American Academy of Ophthalmology, 5 Mar. 2019, www.aao.org/eye-health/glasses-contacts/what-is-prism-correction-in-eyeglasses.

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