What Does Axis Mean for Glasses Prescription?
Eye doctors examine eyes to diagnose vision problems and refractive errors. One aspect that eye doctors consider is the eye axis.
The axis indicates the position of astigmatism in the eyes. It does not indicate the strength of an eyeglass prescription.
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
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.
What is a “Normal” Eye Axis?
Those with a “normal” eye axis will not see CYL 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. The light falls at the right place on their retina.
What Are the Different Types of Prescriptions?
Each eyeglass prescription contains different information, depending on your eyes' condition.
Farsighted prescriptions have a plus sign (+) and a number to show how much lens power is necessary for vision correction. Lens power is represented in diopters.
Eye care specialists may use various tools to determine the degree of farsightedness (distance vision).
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 a person's refractive error
Nearsighted prescriptions have 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 no correction is needed), the stronger the prescription is.
Bifocal Prescriptions (Multifocal)
Multifocal eyewear, such as bifocals or trifocals, are for people with eyeglass prescriptions that have an added value.
This option may appeal 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.
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 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 with 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. 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 show how an actual prism would work. A Fresnel prism is a thin vinyl sticker. It can be used when this type of eye prescription is temporary.
Axis refers to the position of astigmatism in the eyes. The cylinder power (CYL value) is the lens power for astigmatism. If you don't have CYL values listed in your prescription, then you don't have astigmatism. Your eye doctor can determine these numbers during an eye check-up.
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