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How to Read Your Glasses Prescription: The Meaning of Sphere, Cylinder, & Axis

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An optometrist holding a pair of prescription glasses as he smiles and looks directly at the camera.

If you’ve ever received your glasses prescription and been completely confused by all the terms and numbers, you’re not alone. Reading and interpreting your prescription can be pretty intimidating, but knowing about all the measurements taken during eye exams can help you understand why they’re so vital for getting the right prescription glasses.

Sphere (SPH) refers to the lens power needed to help you see clearly, cylinder (CYL) is used to measure the degree of astigmatism affecting your eyes, and axis is used to indicate how astigmatism lines up with your cornea and lenses. 

Altogether, these measurements can be used to create glasses with lenses customized for your vision needs.

The Abbreviations & Terminology

The first thing to note is that a glasses prescription can have multiple rows or columns of information: one for distance vision (commonly labeled “OD” for the right eye and “OS” for the left eye) and another for close-up vision that’s often used for bifocal and multifocal lenses (often abbreviated as “ADD”).

The 3 measurements you’ll often see in each row are sphere (SPH), cylinder (CYL), and axis.

What Does Sphere Mean?

Sphere indicates the strength of the lens power needed to correct nearsightedness (myopia) or farsightedness (hyperopia). The number can be positive (+) or negative (-). A positive sphere measurement is used to indicate someone is farsighted, while a negative sphere measurement is used for nearsightedness. 

What Does Cylinder Mean?

The cylinder component of your prescription is used to measure the strength of astigmatism affecting your vision. 

Astigmatism typically occurs when the front of your eye is less round and slightly shaped like a football or when your eye’s natural lens has curves that affect your vision. The cylinder number relates to your degree of astigmatism, with a higher number indicating more severe astigmatism. 

Astigmatism is corrected with a lens designed to compensate for uneven curves in your cornea or the natural lens in your eye.

What Is the Axis on Your Glasses Prescription?

The axis measurement in your prescription refers to the exact location on your lens where the power used to correct astigmatism will line up with the astigmatism on your cornea. 

Axis is expressed in degrees, ranging from 0 to 180, based on a meridian line from the center to the edge of your lens that contains no cylinder power. Knowing the proper axis is crucial for ensuring your lenses are correctly aligned.

Why Is Your Contact Lens Prescription Different?

If you’re a contact lens wearer, you likely leave your eye exams with two separate prescriptions, including one specifically for your contact lenses. Because contact lenses are placed on the eye, while glasses sit in front of the eye, your prescription for contact lenses must take the change in refractive distance and the shape of your eye into account as well.

Sphere in Contact Lenses

A contact lens prescription measures power from the back surface of the lens to the front surface of the eye. Since contact lenses sit directly on the eye, refractive power needs to be compensated differently to help you achieve clear vision.

Curve Measurement for Contact Lenses

The curve of the lens is another factor that can affect your prescription. Contact lenses are curved, while glasses lenses are flatter. The curve of a contact lens must be fitted precisely to the eye, so the prescription must compensate for the difference between the eye’s curves and the lens.

Contact Lens Fitting

During a contact lens fitting, your eye doctor will take additional measurements, such as corneal curvature and pupil size, to get a proper fit for your lenses. These measurements can affect your prescription, which is another reason why contact lens prescriptions differ from glasses prescriptions.

Another difference worth mentioning is that contact lens prescriptions include the type of contact lens, its brand, and its diameter. This information is necessary for ordering contact lenses that suit your lifestyle and daily vision needs.

A young woman smiling and trying on glasses in a store while being assisted by an optician or optometrist.

Visit Total Vision La Mesa for Your Corrective Prescriptions

Deciphering your glasses prescription can be intimidating at first. While it’s not essential for you to understand all the numbers and terms, it’s interesting knowledge you can use to communicate with your optometrist.

Whatever your vision correction needs are, you can contact us at Total Vision La Mesa to receive an accurate prescription tailored to your unique situation. During your visit, we can take the time to explain your specific prescription and answer any questions you may have about the measurements we take.

Written by Total Vision

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