Vision Information - Eyeglass Lens Prescription Explained

Vision Information – Eyeglass Lens Prescription Explained

How do you read an eyeglass lens prescription? An eyeglass lens prescription may look like a bunch of technical jargon to some, but it’s actually a very precise set of directions that is easy to read for vision industry professionals. Prescriptions paper designs may come in different formats, but they are all written in a universal language that most eye doctors can read.

Definitions of the Terms

Here are some of the terms and their definitions that are commonly seen on an eyeglass prescription:

O.D. – Oculus Dexter. Latin for the right eye.

O.S. – Oculus Sinister. Latin for the left eye.

SPH – Spherical Refractive Error. Lens measurement needed to correct vision.

CYL – Refers to the amount of astigmatism in the eye.

AXIS — Refers to the direction of astigmatism.

PD – Pupil Distance. The distance between the pupils.

PRISM – Lens adjustment needed for eyes to work together.

BASE – Indicates the direction of the PRISM measurement.

How the Terms Work

O.D. and O.S. are two separate fields on the prescription. By differentiating between the right and left eye, a doctor can write a separate prescription for each eye on one page. The rest of the terms below are used to describe the prescription for the left and right eyes.

SPH is based on the eyeball’s inability to correctly focus light. By augmenting the lens convex or concave, the SPH measurement can focus light correctly on the retina. SPH lens power is measured in diopters, and denotes the amount of correction needed to restore optimal vision. If the number is negative, it means nearsightedness, a positive number denotes farsightedness.

CYL is the lens power needed to correct astigmatism. Since astigmatism is the unfocused direction of the light to the retina, it causes the light to be off-center in various degrees. 

AXIS is the degree of where the light hits your eye after entering the pupil. The AXIS and CYL work together to produce a lens augmentation that corrects the astigmatism. If you do not have astigmatism you will not see an entry for the AXIS or CYL field.

PD is the millimeters between the pupils. This measurement is necessary to ensure that the center of both lenses align with the center of the respective pupils. By aligning this point on both lenses, the glasses can create optimal vision for the wearer.

PRISM is another diopter measurement that is used to stabilize the cooperation of both eyes. This measurement is only used when the patient has been diagnosed with a form of strabismus, which is not very common.

BASE describes the direction of the PRISM power, therefore a number for this field will only be entered when a PRISM number is entered.

Your unique lens prescription combines these features to build a pair of glasses that will be optically best for you. The prescription may also have an expiration date, lens type, and other fields, but these are optional by the doctor. By using these universal terms to describe ocular imperfections, prescriptions can be interpreted by eye doctors around the world.


Related – Vision Patients: Asking the Right Questions, Medical Reasons for Wearing Sunglasses, Presbyopia, Eye Anatomy: Important Definitions, Pupil Distance – How to Measure Your Pupillary Distance (PD) .

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