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What do all those numbers on my eyeglass prescription mean?

An eyeglass prescription is written in a standardized format so it can be interpreted worldwide. Let's look at one and see what all the numbers mean: Let's say the eyeglass prescription for your right eye is: -2.00 -1.00 x 180.
The first number (-2.00) indicates the spherical lens power to correct nearsightedness or farsightedness. If the number is preceded by a minus sign (as it is here), this indicates a lens power to correct nearsightedness. If the number were preceded by a plus sign (+), then the lens power would be one to correct farsightedness. In all cases, the unit of power for numbers in an eyeglass prescription is called a diopter (D). So this eyeglass prescription is calling for the correction of 2.00 D of nearsightedness.


The second number (-1.00) is the supplemental lens power (called "cylinder" power) being prescribed for the correction of astigmatism. Cylinder power can be preceded by a minus sign or a plus sign, depending on the prescribing style your eye doctor chooses. If you have no need for astigmatism correction, your doctor might simply draw a horizontal line through this box on your prescription or write "SPH" or "DS" to indicate that only spherical power to correct nearsightedness or farsightedness is needed.


The third number ( x 180) indicates the location of what's called the "axis" of the astigmatism correction. Unlike spherical lens powers that correct nearsightedness or farsightedness, cylinder powers to correct astigmatism have a different amount of power in different meridians of the lens. (Think of meridians like the spokes of a wheel.) The axis of astigmatism is the meridian of the cylinder that has zero power, and it always will be a number between 1 and 180 on your prescription, preceded by an "x." If you have no astigmatism, this part of your eyeglass prescription will be left blank.


If you have presbyopia and need bifocals or other multifocal lenses, your eyeglass prescription will contain a number in a box marked "ADD". This is the additional magnifying power placed in the lower half of your lenses to improve your reading vision. It might be preceded by a "+" sign, and it typically will be a number between 0.75 and 3.00.

Finally, you may see the notations "OD" and "OS" on your eyeglasses prescription. These are abbreviations for Latin terms that mean "right eye" (OD) and "left eye" (OS). Sometimes, you might see a third abbreviation: "OU." This means "both eyes." 


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