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Understanding Prescription


You noticed that you can not see well and you went to eyes check, you doctor gave you  a prescription for lenses. On that you can see a lot abbreviated terms as OD, OS, SPH and CYL, but what this really mean?


If you want to understand your prescription first you need to know what "OD"“OS" and "OU" mean. They are abbreviations for oculus dexter, oculus sinister and oculus uterque which are Latin terms for right eye and left eye, and "both eyes."

On some prescriptions you may find RE (right eye) and LE (left eye) instead of OD and OS, some doctors and clinic wanted to simplify their prescription.


The information for your right eye (OD) comes before the information for your left eye (OS) this is because your doctor write prescriptions face to you, so he/she see your right eye on their left (first) and your left eye on their right (second).


One sample example, if your prescription says -1.00, you have one diopter of nearsightedness. This is a fairly mild amount of nearsightedness. If you are -4.25, that means you have 4 and 1/4 diopters of nearsightedness. This is more nearsighted than -1.00, and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness and +5 would be more.

If your prescriptions has abbreviations S x C x Axis that’s mean you have asigmatisam. The S refers to the "spherical" portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above. The C refers to the "cylinder" or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that you have. The more astigmatism you have the bigger this number will be.

Other Terms On Your Prescription

Sphere (SPH). Is correcting nearsightedness or farsightedness and indicates the amount of lens power, measured in diopters (D), prescribed to you. If the number appearing under this heading has a minus sign (–), you are nearsighted; if the number has a plus sign (+) or is not preceded by a plus sign or a minus sign, you are farsighted.

The term "sphere" means that the correction for nearsightedness or farsightedness is "spherical," or equal in all meridians of  the eye.


Cylinder (CYL). The term "cylinder" means that this lens power added to correct astigmatism is not spherical, but instead is shaped so one meridian has no added curvature, and the meridian perpendicular to this "no added power" meridian contains the maximum power and lens curvature to correct astigmatism.

The number in the cylinder may be preceded with a minus sign (for the correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism). Cylinder power always follows sphere power in an eyeglass prescription.

Meridians of the eye are determined by superimposing a protractor scale on the eye's front surface. The 90-degree meridian is the vertical meridian of the eye, and the 180-degree meridian is the horizontal meridian.

Here are two examples of what prescriptions for eyes with astigmatism could look like:

-3.00 +0.50 x 180

+1.50 +2.00 x 60

The first prescription means that the person has 3 diopters of nearsightedness on one eye with 0.5 diopters of astigmatism and an axis of 180 degrees on another.

The second prescription means that the person has 1.5 diopters of farsightedness on one eye, 2 diopters of astigmatism and an axis of 60 degrees on another.


Axis. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.

If an eyeglass prescription includes cylinder power, it also must include an axis value, which follows the CYL power and is preceded by an "x" when written freehand. The axis is the lens meridian that is 90 degrees away from the meridian that contains the cylinder power.