An eyeglass prescription is a set of separate measurements, not one score for how well you see. Read the right and left eye in their own rows, then interpret each abbreviation according to its job: SPH gives the main lens power, CYL and AXIS work together for astigmatism correction, and ADD or prism describe additional corrections when included.
Start with the eye labels and prescription layout
Most prescriptions identify the right eye as OD and the left eye as OS. These abbreviations come from Latin terms for right and left eye; OU, if it appears, means both eyes. The numbers in each eye’s row belong to that eye, and the two rows may differ. Avoid reading across a page as if every value were a single sequence shared by both eyes.
SPH means sphere, the main lens power, measured in diopters. A minus sign generally indicates a correction for nearsightedness, while a plus sign generally indicates a correction for farsightedness. The sign matters: -2.00 and +2.00 are not interchangeable values. The number gives the power specified for that lens component; it is not a percentage of vision or a rating of eye health.
Forms vary in how they display empty or inapplicable fields. Some may use a dash, leave a space, or show a notation such as “DS” for sphere-only correction. The layout is a record of the prescribed lens values, so interpret the labels and entries together rather than assuming that every column must contain a number.
Read CYL and AXIS as a pair
CYL, short for cylinder, records the amount of astigmatism correction in the prescription. Like SPH, it may carry a plus or minus sign. AXIS records the orientation of that cylindrical correction in degrees, from 1 through 180. Axis is an angle, not another measure of lens strength; it only has meaning in relation to a cylinder value.
Consider an illustrative right-eye row with SPH -2.00, CYL -0.75, and AXIS 180. The sphere is a minus power, and the cylinder adds a specified amount of astigmatism correction at an orientation of 180 degrees. Treating all three entries as three strengths would confuse two different kinds of information: CYL provides a power, while AXIS tells how that correction is oriented.
These fields can be compared at a glance:
| Field | What it describes | How to read it |
|---|---|---|
| SPH | Main lens power | Read the value and its plus or minus sign |
| CYL | Amount of astigmatism correction | Read its power and sign |
| AXIS | Orientation of cylinder correction | Read as degrees, not lens power |
Recognize ADD and prism values
ADD is additional lens power for near viewing, commonly included for multifocal designs. It supplements the distance prescription rather than replacing SPH. A prescription may show one ADD value that applies to both eyes, or present the information in a layout that associates values with each eye. Its presence tells the lens maker that the near portion has an added power component.
Prism is a different kind of entry. It specifies a correction that shifts the apparent position of an image. The amount describes the prism correction, while a base direction describes its orientation. Those are distinct pieces of information: a direction such as up, down, in, or out is not itself a power value. Prism entries may appear only on prescriptions that include this correction.
Use the prescription for the right purpose
Use each label to understand what was written, not to independently decide whether a prescription is suitable for you. For example, a minus SPH does not, by itself, describe every detail of how your eyes function; it is one lens value within a larger prescription. The complete combination of eye, power, orientation, and any added correction is what guides the specified eyeglass lenses.
An eyeglass prescription is not a contact-lens prescription. Contact lenses sit directly on the eye, so fitting information such as base curve and diameter, along with the particular lens product, matters in addition to vision correction. The two prescriptions are not interchangeable, even when they address similar vision needs.
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