Lenses

By: Carmen Willings
teachingvisuallyimpaired.com

Reports should indicate what a student's uncorrected (without glasses) and corrected (with glasses) visual acuities are. If a student's vision can be corrected, even mildly, they will typically be prescribed glasses or contacts. ​

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.

A convex, or plus lens

A convex lens bulges outward, converges light rays and increases the size of an image being viewed. A plus lens directs light rays entering the eye so they focus on the retina instead of behind the retina as convex lenses correct hyperopia, a condition in which the eye is underpowered. Lens power that is greater than +12D could mean that the student has no crystalline lens (due to cataract surgery or aphakia) or the student has extreme farsightedness.

A concave, or minus lens

A concave lens bulges inward, diverges light rays and decreases the size of the image received or can be used to converge light rays so that they focus on the retina, rather than in front of it. A very strong minus lens indicates extreme nearsightedness, also referred to as high myopia.

Plano lenses

A plano lens does not bulge inward or outward and is not for correction in eyeglasses. A student may be prescribed plano lenses if they have no vision in one eye and the lens is used to "balance" the prescription for the other eye. It may also be used for cosmetic reasons or for safety reasons.

Prism Lenses

Prism lenses are often prescribed for persons who have strabismus for the purpose of redirecting the rays of light entering the eye. Prisms may also be used for persons with visual field neglect to cue the person that additional information exists in the neglected field.

​Other Terms...

  • The sphere is the amount in diopters of correction to sharpen vision to best correction.
  • The cylinder power is the amount of lens power needed to correct astigmatism.
  • The axis is a number that indicates location of cylinder power in lens for astigmatism. 
  • Prism is added to some prescriptions to compensate for eye alignment problems.
  • Base refers to the location of the thickest edge of the prism and direction it redirects light. 

Sample Eyeglasses Prescription

 
 
Sphere
Cylinder
Axis
Prism
base
Distance
OD
-3.75
 
 
0.5
down
 
OS
-4.25
-0.50
180
0.5
up

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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