Understanding your Prescription

During your visit to the eye specialist – an optometrist or ophthalmologist, he or she provides you an eyeglass prescription. The prescription lets you know if you're near or farsighted, and also if you have an astigmatism or presbyopia. The prescription includes various abbreviations and numbers that are certainly not easy to understand for the casual reader. However, you need not worry. We have all the answers right here for you.
Generally the very first term on your prescription, Distance refers to the lens power needed to see distant objects clearly, and is determined when you are requested to try various lenses while reading the eye chart.
The most important terms in your eyeglass prescription are OD and OS, short for Oculus Dexter (a Latin term that means right eye) and Oculus Sinister (Latin for left eye) respectively.  You may also find the term OU on the prescription, which is the acronym for Latin Oculus Uterque, meaning both eyes.
Though not as prevalent in use, some eye specialists also tend to use RE for the right eye and LE for the left, in their prescriptions.
Other terms in your prescription include:

SPH:

SPH (Sphere) specifies the amount of lens power prescribed to correct nearsightedness or farsightedness. If the number has a negative sign (–) with it, you are nearsighted, and if it has a positive sign (+) or no sign whatsoever, you are farsighted.

CYL:

CYL (Cylinder) shows the amount of lens power required in case of astigmatism. The number may have a negative sign (for nearsighted astigmatism) or a positive sign (for farsighted astigmatism). If there is no number, you have no astigmatism, or your astigmatism is minimal enough to be discounted.

Axis:

Axis indicates the lens meridian which contains no cylinder power for correcting astigmatism. It is described through a number from 1 to 180, with 90 standing for the vertical meridian of the eye while 180 denoting the horizontal meridian. Prescriptions that include cylinder power also include an axis value

Prism:

Prism describes the amount of prismatic power, measured in prism diopters, prescribed for eye alignment problems. This value is followed by the base, indicating which edge of the prism will be thickest: BU (base up), BD (base down), BI (base in), or BO (base out). Most eyeglass prescriptions though do not contain prism.

Add:

Add signifies the added magnifying power imbued in the lower part of multifocal lenses for correcting presbyopia. Add Power, as the name implies, is always a positive number, generally ranging from 0.75 D to 3.00 D and is the same for both eyes.

Sphere Power, Cylinder Power and Add Power are always written in diopters, in decimal form and usually as multiples of 0.25 D. Axis values are whole numbers from 1 to 180, whereas Prism diopters are in decimal form, mostly up to one digit.

Remember that the prescriptions for eyeglasses and contact lens are not at all the same. An eyeglass prescription is meant to be used for the purchase of eyeglasses only, and does not contain certain data that is necessary for the purchase of contact lens.
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There are several things to understand about the Prescription Eyewear Frames your eye doctor has written out especially for you after conducting several exams to find your perfect fit. On your Prescription Safety Eyewear you can find terms such as OD, OS and OU, there refer to your eyes. OD is short for oculus Dexter a Latin term for your right eye, OS means oculus sinister a Latin term for your left eye and finally OU is Latin for Oculus Uterus which refers to both your eyes. SPH or sphere would refer to the amount of lens correction power required for your near slightness or farsightedness. CYL or cylinder is the amount of lens power required for your case of astigmatism.