The eye axis, also known as the ocular axis or visual axis, is a line that passes through the center of the eye and represents the path that light takes as it enters the eye and focuses on the retina. This line typically extends from the cornea (the transparent front layer of the eye) to the lens (which serves to focus light onto the retina) to the fovea (the central area of the retina responsible for sharp central vision).
The eye axis is essential for comprehending the optical properties of the eye and for diagnosing and treating refractive errors such as myopia, hyperopia, and astigmatism. In addition, this scale is used in the fitting and prescription of spectacles, contact lenses, and the planning of refractive surgeries such as LASIK.
How Does the Eye Axis Scale Work?
The eye axis scale is a parameter used in eyeglass prescriptions to determine the type of prescription needed to correct an astigmatism, or a reflective error that is caused by an abnormally shaped lens or cornea. This leads to a distortion in the manner in which light is focused on the retina.
In a prescription for eyeglasses, the axis is a degree measurement from 1 to 180 that denotes the orientation of the astigmatism correction in the lens. The axis is only relevant if the prescription contains a cylinder value, as the cylinder value represents the quantity of astigmatism correction required. This ensures that the patient’s vision is correctly corrected.
The eye axis scale is applied as follows.
- The axis is measured from 1 to 180 degrees, with 90 degrees representing vertical and 180 degrees representing horizontal.
- The axis value instructs the lens manufacturer on how to align the cylindrical correction in the lens with the astigmatic meridian of the patient’s eye.
- The axis value is combined with the cylinder value (which specifies the correction power) to produce a lens that effectively corrects the astigmatism of the patient.
The axis value in an eyeglass prescription is unique to each patient and must be measured accurately by an optometrist or ophthalmologist during an eye examination. Incorrect axis alignment in corrective lenses can cause blurred or distorted vision, eye strain, and even migraines.
What Is a Normal Eye Axis Prescription?
There is no specific “normal” prescription for eyeglasses, as they differ significantly based on an individual’s unique refractive errors and visual needs. However, a prescription with low to moderate levels of correction may be considered to be the most common.
Typically, a prescription for eyeglasses includes the following parameters:
- Sphere: This value denotes the amount of lens power required to correct nearsightedness (myopia) or farsightedness (hyperopia). The unit of measurement is diopters (D). Positive values indicate hyperopia, while negative values indicate myopia. The range of low to moderate levels of correction would be -3.00 to +3.00 diopters.
- Cylinder: This value is also measured in diopters and used to correct astigmatism. A negative or positive value indicates the power required to rectify the irregular curvature of the lens or cornea. Low to moderate astigmatism corrections may lie between -2.00 and +2.00 diopters.
- Axis: As stated previously, the axis value represents the orientation of the astigmatism correction in the lens. This is only relevant when the prescription contains a cylinder value. The axis is measured in degrees between 1 and 180.
- ADD or near addition: This value is used for individuals with presbyopia, an age-related decline in near vision that typically begins around age 40. ADD indicates the additional lens power required for close-up duties, such as reading. Typically, it is a positive number ranging from +0.75 to +3.00 diopters or more.
- Prism and base: These values are used to address eye alignment issues or binocular vision issues, such as strabismus or double vision, which are usually less common in prescriptions. Prism is measured in prism diopters, and the prism’s base denotes its orientation (base up, base down, base in, or base out).
What constitutes a normal prescription for eyeglasses may include low to moderate levels of correction for myopia, hyperopia, or astigmatism, and possibly an ADD value for presbyopia. However, it warrants noting that prescriptions are extremely individualized and vary significantly between each person.
How to Read Your Eyeglass Prescription
You can read your eyeglass prescription using the metrics described above. Additionally, when you examine your eyeglass prescription, the parameters for each eye will be listed — oculus dexter (OD) and oculus sinister (OS). Typically, the values for sphere, cylinder, and axis are listed first, followed by any additional values for ADD or prism/base.
For instance, a sample prescription might appear as follows:
OD: -2.00, -1.25, 90, +1.50
OS: -1.75, -1.00, 85, +1.50
This prescription here would indicate that the right eye (OD) has -2.00 diopters of myopia (nearsightedness), -1.25 diopters of astigmatism correction with an axis at 90 degrees, and +1.50 diopters of presbyopia. The left eye (OS) has a myopia correction of -1.75 diopters, an astigmatism correction of -1.00 diopters with an axis of 85 degrees, and a presbyopia ADD value of 1.50.
What Are the Types of Eyeglass Prescriptions?
Doctors typically write eyeglass prescriptions to treat various vision problems and refractive errors. Medical professionals classify the primary varieties of eyeglass prescriptions according to the specific vision problem they are intended to correct.
Single Vision Prescriptions
These are the most prevalent type of eyeglass prescriptions and used to correct a single vision problem, such as myopia, hyperopia, or astigmatism. Single vision lenses are designed to provide distinct vision at a single distance, whether it be near, intermediate, or far.
Bifocals are designed to rectify presbyopia, an age-related decline in near vision that typically begins between the ages of 40 and 50. Bifocal lenses have two distinct zones of lens power: one for distance vision (typically the upper portion of the lens) and one for near vision, which is usually the lower part of the lens. In addition to the standard sphere, cylinder, and axis values, doctors include the ADD value in bifocal prescriptions.
Similar to bifocal prescriptions, trifocals are used to rectify presbyopia. Trifocal lenses, on the other hand, have three distinct areas of lens power: one for distance vision, one for intermediate vision (such as computer use), and one for near vision. Like bifocal prescriptions, trifocal prescriptions will include the ADD value.
Progressive lenses, also known as no-line bifocals or multifocal, are an additional option for individuals with presbyopia. Progressive lenses provide a seamless transition between various lens powers for distance, intermediate, and near vision, whereas bifocal and trifocal lenses have distinct lines between the different lens powers. In addition to the standard sphere, cylinder, and axis values, a progressive prescription includes an ADD value.
Some eyeglass prescriptions may include a prism correction to resolve issues with eye alignment or binocular vision, such as strabismus or double vision. Prism prescriptions will include a prism value measured in prism diopters and a base value denoting the prism’s orientation (base up, base down, base in, or base out).
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Last Updated May 24, 2023
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