Eye Prescriptions (Meaning of SPH, CYL, AXIS)

What Do the Abbreviations on My Glasses Prescription Mean?

During an eye exam, your doctor will assess your eye health and conduct tests to determine if you have any refractive errors. If you do, they’ll likely recommend vision correction.

Some common refractive errors that require treatment include:

Depending on the diagnosis, your doctor will give you an eyeglasses prescription containing diagrams and numeric values.

Eyeglasses Prescription Abbreviations Explained

Your eyeglasses prescription contains crucial pieces of information about your vision. Keep in mind that your eyeglasses prescription isn’t the same as your contact lens prescription.

By the end of this article, you’ll know exactly how to read your glasses prescription and what the abbreviations and numbers mean.

OD & OS

You’ll typically find the abbreviations OD and OS on the left side of your prescription. These are abbreviations for the Latin terms oculus dexter and oculus sinister.

OD refers to the right eye, while OS refers to the left eye. You may also notice a column labeled OU, which means oculus uterque, or both eyes.

Some eye doctors have replaced OD with RE (right eye) and OS with LE (left eye).

SPH

Sphere, sometimes abbreviated as SPH, indicates the lens power needed to correct nearsightedness, farsightedness, or both. The number indicated by this abbreviation refers to the lens power you’ll need to correct these flaws in vision.

The term “sphere” refers to the fact that distance vision correction is spherical or equal in all meridians of the eye. Here’s what to know about SPH on your eyeglasses prescription:

Meridians of the eye are imaginary lines that pass through the pupil's center, measured by a protractor superimposed over the eye's surface.
SPH is measured in diopters, the unit of measurement for the lens's refractive (light-bending) power.
If the number is negative (–), the prescription corrects nearsightedness.
If the number is positive (+), it corrects farsightedness.
A number between +/-.025 to +/-2.00 is considered mild, a number between +/-2.25 to +/- 5.00 is considered moderate, and a number greater than +/- 5.00 is considered severe.

CYL

Cylinder, abbreviated CYL and measured in diopters, indicates the lens power needed for astigmatism correction. Astigmatism is an uneven curvature of the eye’s cornea or lens.

If this column is blank, you have no astigmatism. A minus sign (–) means your prescription corrects nearsighted astigmatism, while a plus sign (+) corrects farsighted astigmatism.

Cylinder correction power always follows sphere power in an eyeglass prescription.

Axis

The axis number indicates the degree of the angle at which the cylinder is placed in your corrective lens. The figure is used to correct astigmatism. If a prescription includes cylinder power (CYL), it must be followed by an axis.

The axis value highlights the angle of orientation in degrees, with a number ranging from 0 to 180. The value is often preceded by an “x” when written in freehand.

Add

Add represents Addition and indicates the magnifying power applied to the bottom part of bifocal, multifocal, or progressive lenses used to correct presbyopia (age-related farsightedness).

This number is always positive, even if it doesn’t have a plus sign (+) in front of it. The value is typically the same for both eyes and ranges from +0.75 to +3.00.

Prism

Prism correction is used in eyeglasses for people with eye alignment issues, such as diplopia (double vision) or strabismus (eye turn).

The prism number is measured in diopters and appears as a decimal (0.5) or fraction (½) followed by two letters indicating the direction of the base, or thickest edge, of the prism:

BU stands for "base up."
BD stands for "base down."
BI stands for "base in."
BO stands for "base out."

Additional Eye Prescription Information

Your optometrist or ophthalmologist may include additional information regarding your eye health on your prescription.

Medical eyeglass prescription with parameters

Additional information usually refers to specific types of corrective lens recommendations, such as:

Multifocal Lenses. Multifocal lenses have multiple focusing powers. This improves your vision at different distances. Bifocal and trifocal glasses feature a sharp edge between the close-up and far-off part of the prescription.
Progressive Lenses. Progressive lenses correct near, far, and middle vision with a seamless transition in magnification from top to bottom.
Anti-reflective Coating. Anti-reflective coating have a special coating applied to the lenses that reduces glare that occurs when light hits the lenses. Reduced glare leads to better visual clarity and less eye fatigue.
Photochromic Lenses. Photochromic lenses stay transparent indoors, but darken when exposed to ultraviolet light from the sun.

Some eyeglass prescriptions may require a PD (pupillary distance) measurement.

How Do Contact Lens Prescriptions Differ From Glasses Prescriptions?

Because contact and eyeglass lenses perform the same function for the eyes, many people believe their prescriptions would be the same.

However, this isn’t the case. Glasses sit about 12 mm from the eyes, while contact lenses sit directly on the eye’s surface. The distance between the lenses can drastically change prescription strength.

Additionally, because contact lenses sit directly on the eyes, prescriptions require further specifications that glasses prescriptions do not, including:

Lens diameter. The lens diameter specifies the lens size measured by your eye. Lens diameters are not one-size-fits-all and require a fitting exam.
Base Curve. The base curve is the curvature of the back lens and is determined by the shape of your cornea. The base curve determines the fit of your lens and ensures it stays in place.
Lens Brand. Each brand uses different lens materials, determining breathability and comfort.
Expiration Date. Contact lenses and glasses prescriptions typically have different expiration dates. Typically, contact lens prescriptions expire before glasses prescriptions do.

What Should I Expect During an Eye Exam?

During an eye exam, your provider will perform several tests to check your vision and evaluate your eye health.

Standard tests during an eye exam can vary but typically include:

Visual Acuity. Visual acuity refers to clarity or sharpness of vision. This test tells your provider if you need glasses and helps find the right corrective lens prescription.
Visual Field. Your provider will check your peripheral vision by holding up a finger or object and moving it from one side of your face to another. This test tells providers about your full range of vision.
Color Vision Test. Your provider will show you images with colored dots. Subtle color differences in the dots will reveal numbers. People with color blindness may not be able to see the numbers.
Corneal Topography. This test shows a curvature of the cornea to fit contact lenses or prepare for corneal transplants and other eye surgery.
Ophthalmoscopy. Your provider may use eyedrops to dilate your pupils. They will shine a light in your eye to examine your cornea, lens, retina, optic nerve, and surrounding blood vessels.
Tonometry. Your provider will use a tonometer to blow a small puff of air onto the cornea. The test detects problems with pressure in the eye. Increased pressure is a sign of glaucoma.⁸

Summary

An eye prescription specifies the values needed for your corrective eyewear. Your prescription will usually contain the following information:

OD & OS
SPH
CYL
Axis
Add
Prism

A prescription for glasses is different from a contact lens prescription. Your optometrist or ophthalmologist will perform an eye examination to determine the necessary values to correct your eyesight and assess your eye health.

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