Understanding Your Eye Prescription

Making sense of your prescription after an eye exam is akin to deciphering code written in a foreign language. Eye doctors, like optometrists, will give you your eye prescription using certain abbreviations and specified measurements. Here’s a quick guide on how to understand them:


Some professionals, especially those in the optometry field, will simplify by writing a prescription using LE and RE -left and right eye- however, this is not always the case. The first thing you need to understand are the abbreviations that come from Latin.

The acronyms OS and OD respectively refer to Oculus Sinister and Oculus Dester. Oculus means eye and sinister, left, whereas dester is right. When referring to both eyes, the Latin term is Oculus Uterque, or OU for short.

If all that left you tongue tied and even more confused, the following will make things much easier to understand and remember.

  • LE or OS (Left Eye): Refers to your left eye prescription
  • RE or OD (Right Eye): Refers to your right eye prescription
  • SPH (Sphere of the eye): The lens' ability to refract light
  • CYL (Cylinder of the eye): Correction number needed for astigmatism
  • Diopters: Measurement units used to determine the optical power of the lens
  • ADD Measurement: This is your reading addition and relates to the amount of additional correction needed to focus at close distances.


Now that we’ve got that down, as you go on to read your eyeglass prescription, you will see abbreviations like SPH and CYL. First, SPH refers to the sphere of the eye, and the lens’ ability to refract light.

Farsightedness (problems with seeing objects up close) is a positive number, or one without a plus or minus sign. If the number is negative, this indicates nearsightedness (the inability to see objects clearly at a distance).

Diopters are the measurement units used to determine the optical power of the lens, as well as its curvature, both of which are based upon a scientific formula that determines focal length, or the ability of the eye to focus. This corrective number, if large, means that your nearsightedness or farsightedness needs more correction. If you see SPH, this tells you that the number is spherical---equal in all the sectors of the eye’s measurements. For example, if your OD has an SPH of -7.25, then this means that your right eye needs 7.25 diopters of correction as this eye is myopic (nearsighted). Lenses that are concave (when the surface curves inwards) are made to correct this. Convex (when the surface curves outwards) lenses are used to correct hyperopia, or farsightedness, and presbyopia, the condition that comes with advanced age whereby adults need corrective lenses in order to read due to the reduction of elasticity in the eye.

Next, the acronym CYL refers to the cylinder of the eye, and is the correction number needed for those with astigmatism, meaning that either the cornea in front of the eye is misshapen or the lens at the back of the eye does not curve and change the direction of light properly, or both. This results in blurred vision if not corrected with prescription glasses. As such, CYL adds an extra item of correction to amend this.

There is also a number that refers to the place in the eye where the astigmatism or difference in refraction is. This is called the Axis. It is a number between 0 and 180 with 90 being the vertical location line in the eye and 180 the horizontal one. Another measurement value is the prism, which may also be present on your prescription that is used to correct eye alignment. This is, however, rare.

Lastly, an ADD measurement may appear on bifocal prescriptions to correct presbyopia instead of prescribing separate reading glasses. Bifocal lenses will have two delineations or sections on the lens: one for near vision and one for distance vision Progressive lenses, on the other hand, make a gradient shift on the lenses. This type of lens takes some getting used to, but many patients experience better comfort and success with them.

Prescriptions for Contact Lenses

It is important to remember that as contact lenses rest directly on the eye, they must be properly fitted by a professional. The measurements will have some of the same acronyms, but will be based upon a correction that is not a few inches away from the wearers eye. Bad reactions to different components of the lenses may be noted in the fitting process and prescriptions are also likely to reflect material and length of wear guidelines. Custom fitting and measurements will reflect this on a contact lens prescription as each patient is different as in terms of how comfortable the lenses are (or aren’t) and varied lifestyles.

If you’re still having trouble interpreting your prescription, don’t try to guess and speak to a professional.

Questions about your eye prescription?

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