Demystifying the Components of Your Glasses Prescription

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Have you ever wondered what those mysterious numbers and abbreviations on your glasses prescription mean? If so, you’re not alone! A glasses prescription can look like a jumble of letters and numbers, but it holds the key to clear and comfortable vision. In this blog post, we will decode the various parts of a glasses prescription, helping you understand what your optometrist or ophthalmologist is really telling you.

OD and OS

In a glasses prescription, “OD” represents the right eye, derived from the Latin term “oculus dexter,” and “OS” denotes the left eye, derived from “oculus sinister.” These abbreviations are used to clearly distinguish between the prescription requirements for each eye.

Sphere (SPH)

The first part of your prescription is often labeled as “SPH” or “Sphere.” This component indicates the amount of nearsightedness (negative number) or farsightedness (positive number) you have. The magnitude of this number represents the strength of the lens needed to correct your vision. If your prescription reads -2.00, you are moderately nearsighted, while +1.50 indicates moderate farsightedness.

Cylinder (CYL)

The cylinder value, abbreviated as “CYL,” is crucial for individuals with astigmatism. Astigmatism occurs when the cornea or lens of the eye is not perfectly spherical, causing distorted or blurry vision. The cylinder value indicates the degree of astigmatism correction required. It is always accompanied by an axis value, which determines the orientation of the astigmatism correction.


The axis is a measurement in degrees and is associated with the cylinder value. It specifies the angle at which the astigmatism correction should be applied to your lenses. The axis value ranges from 0 to 180 degrees, with 90 degrees representing vertical astigmatism and 180 degrees representing horizontal astigmatism.

Add (ADD)

The “ADD” value is typically found in prescriptions for multifocal or bifocal lenses. This component is used to correct presbyopia, a condition that affects near vision as we age. The ADD value represents the additional magnifying power needed in the lower part of the lens to help you read or see up close.

Pupillary Distance (PD)

The pupillary distance, or PD, is not a part of the prescription per se but is a crucial measurement for ensuring that your glasses are properly centered over your eyes. It represents the distance between the centers of your pupils and is measured in millimeters. There are two types of PD: monocular (distance between each pupil and the center of the nose) and binocular (distance between the centers of both pupils).

Prism (PRISM)

The prism value, if present, is indicated in prism diopters (Δ). It is used to correct eye alignment problems, such as strabismus or double vision. The prism may also include a base direction (e.g., BU for base up or BD for base down) to specify the orientation of the correction.

Prism Base Direction

If you have a prism correction, your prescription may include a base direction, as mentioned earlier. This indicates the direction in which the prism is needed to correct any eye alignment issues. It can be BU (base up), BD (base down), BI (base in), or BO (base out), depending on the issue being corrected.

Understanding the various components of your glasses prescription is essential for ensuring that you receive the correct eyewear to correct your vision problems effectively. The sphere, cylinder, axis, and ADD values are the core elements that address refractive errors like myopia, hyperopia, and astigmatism. If your prescription includes prism, it’s aimed at managing eye alignment issues. By decoding these parts of your prescription, you’ll be better equipped to communicate with your eye care professional and select the right eyewear to see the world more clearly. Understanding the parts of your glasses prescription can also aid in finding the best pair for you when browsing our collection here at ReSpectacle.

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