​Why were the glasses prescribed?

It is important to know why the glasses have been prescribed. Correcting a refractive error (such as myopia or hyperopia) is only one of the reasons that these may be prescribed. ​Glasses may be prescribed with a prism for a muscle imbalance or they may be recommended to protect the eyes. If the glasses are prescribed for near, they should be removed for distance viewing and vice versa if recommended by the doctor (some students will be able to leave them on as it will not have a significant impact). Glasses that are prescribed for myopia, or nearsightedness, will have a minus lens. Glasses prescribed for hyperopia will have a plus lens.

​Types of Lenses

The type of lenses that are prescribed can tell you a lot about the student's eye condition. The stronger the lens, the more significant the uncorrected vision. Keep in mind that even if a student is prescribed glasses, it doesn't necessarily mean that the student's vision can be fully corrected. This is because glasses cannot correct all types of visual impairments.
Convex (Plus) Lenses
  • Shape: Convex lenses curve or bulge outward in the center and are thicker in the middle than at the edges. This shape is designed to bend incoming light toward a focal point.
  • Function: These lenses converge (bring together) light rays as they enter the eye. By doing so, they help focus images more accurately on the retina. Because of this converging effect, convex lenses also make objects appear larger and clearer, which can improve near and distance vision as needed.
  • Corrects: Convex lenses are primarily used to correct hyperopia (farsightedness), a condition in which a person sees distant objects more clearly than those up close. These lenses provide the extra focusing power needed to bring near images into sharp focus. 
  • Notes: Convex lens prescriptions are written with a plus (+) number in diopters (D). Stronger plus prescriptions, especially those greater than +12.00 diopters, may suggest severe or extreme farsightedness or Aphakia.
Concave (Minus) Lenses
  • Shape: Concave lenses curve inward, appearing thinner in the center and thicker at the edges. Their inward curve gives them a “hollowed” look compared to convex lenses.
  • Function: These lenses diverge (spread apart) incoming light rays. By doing this, concave lenses shift the focal point back onto the retina, helping produce a clear image. Because of this diverging effect, concave lenses make objects appear slightly smaller than they really are.
  • Corrects: Concave lenses are used to correct myopia (nearsightedness). People with myopia can see nearby objects clearly, but distant objects appear blurry. Minus lenses reduce the eye’s focusing power so that distant images can be seen more clearly.
  • Notes: Concave lens prescriptions are written with a minus (–) number in diopters (D). Stronger minus lenses, especially prescriptions beyond -6.00D or -10.00D, can indicate high or severe myopia.High myopia often requires more frequent eye exams, monitoring for retinal changes, and possible use of low vision aids, depending on severity.
Plano Lenses
  • Shape: Plano lenses have a flat surface with no inward or outward curvature. Because they lack curvature, they do not bend or focus light the way corrective lenses do.
  • Function: Plano lenses have no corrective power. They do not change how light enters the eye, so they do not improve or alter vision. Their primary purpose is non-corrective; they serve functional, cosmetic, or protective roles rather than addressing refractive errors.
  • Uses: Plano lenses can be used in several situations:
    • Balance prescriptions. When one eye requires correction, and the other does not (such as in cases of monocular vision or no functional vision in one eye), a plano lens may be placed in the non-seeing eye’s frame. This creates visual and cosmetic balance so the glasses appear symmetrical and sit correctly on the face.
    • Cosmetic purposes. Plano lenses may be used to maintain symmetry in thicker lenses, reduce the appearance of anisometropia (unequal lens powers), and fill a frame when a person wants the appearance of wearing glasses but does not need correction
    • Eye protection. Plano lenses can be made of polycarbonate or other impact-resistant materials to protect the eyes during sports, shop, or lab classes, outdoor activities, or work environments with debris or hazards. 

​Other Prescription Terms

Sphere (SPH). The sphere value indicates the amount of lens power, measured in diopters (D), needed to correct nearsightedness (myopia) or farsightedness (hyperopia).
  • A negative (-) sphere number means the person is nearsighted and sees better up close than far away.
  • A positive (+) sphere number means the person is farsighted and has more difficulty with close-up tasks.
This value tells the optometrist how much the lens needs to bend light to bring images into sharp focus. The higher the SPH number, the stronger the glasses needed to produce the student’s best-corrected visual acuity.
Cylinder (CYL). The cylinder value represents the amount of additional lens power needed to correct astigmatism, which occurs when the cornea or lens is shaped more like a football than a basketball.
  • Astigmatism can make vision blurry or distorted at any distance.
  • The CYL value may be written as a negative or positive number depending on the prescribing method.
If no cylinder value is listed, it means the person has little or no astigmatism.
Axis. The axis indicates the orientation (measured in degrees from 1 to 180) of the astigmatism correction.
  • Astigmatism is not uniform—it has a specific direction where the cornea is steeper or flatter.
  • The axis tells the lens where to position the cylinder power to properly correct that uneven curvature.
For example:
  • An axis of 90° aligns vertically.
  • An axis of 180° aligns horizontally.
The axis does not reflect power; it simply identifies the angle where correction is needed.
Prism. Prism is added to some prescriptions to help correct eye alignment or binocular vision problems, such as:
  • Strabismus (eye turn)
  • Convergence insufficiency
  • Double vision (diplopia)
Prism lenses do not improve clarity—they shift or redirect light so each eye can work together more effectively. This helps the brain combine images from both eyes into a single, stable picture. Prism is measured in prism diopters.
Base. The base describes the direction of the prism's thickest edge, which determines where the image is shifted. Common base directions include:
  • Base Up (BU) – shifts the image downward.
  • Base Down (BD) – shifts the image upward.
  • Base In (BI) – shifts the image toward the nose.
  • Base Out (BO) – shifts the image toward the ear.
The base direction ensures the prism compensates accurately for misalignment, helping reduce eye strain, double vision, or tracking issues. 
A glasses prescription is not just about clarity. It is a detailed map of how to best focus and align light for that individual’s visual system. Understanding SPH, CYL, axis, prism, and base helps educators and families better communicate with eye care professionals and recognize how a student’s eyewear supports their visual functioning in school and everyday activities.

All About Vision

Gary Heiting, OD has written a great article for All About Vision entitled "How to Read Your Eyeglass Prescription." This information can be very helpful when interpreting the eye report!

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