Interpretation
Sphere
The sphere number indicates whether you have nearsightedness or farsightedness.1
Nearsighted people have trouble seeing things that are far away. Farsighted people have trouble seeing things that are up close.
You'll also see a plus sign or minus sign before the number.
A minus sign (-) means you need a negative-powered lens. These lenses are used to correct nearsightedness.
A positive sign (+) means you need a positive-powered lens. These lenses correct farsightedness.
Cylinder
The next number in the sample prescription is -0.50. This is the "cylinder" measurement. On your prescription, "cylinder" might be abbreviated as "CYL."
The cylinder measures the degree of astigmatism in your eye. The number shows how much lens power will be needed to correct astigmatism.1
Axis
The next number is x 180, which is read as "axis 180." On your prescription, "axis" is sometimes abbreviated as "X" or "AX."
This number is an angle in degrees from 0 to 180. If you have astigmatism, this number points to where it is in your eye.
The cornea is the clear covering of your eye. It is the part of your eye that does most of the focusing.
In the example, the ADD number of +1.75 notes how much power needs to be "added" to the distance prescription. This addition helps a person see better for reading and other activities that they do up close.
Younger people's prescriptions usually do not have an ADD number. While some young people can have near-focusing problems, it usually develops as you approach 40.
Some people think the ADD number is the power needed for over-the-counter (OTC) reading glasses (or "readers"), but it's not the same thing.
To get the right number, you need to do a little math.
To find the right reading glasses, add the sphere number to the ADD number.
In the above example, for the right eye, this would be -2.00 and +1.75, which equals -0.25. For the left eye, add +1.00 and +1.75 to get +2.75.
Next, look at the cylinder measurement and the axis measurement.
In the example, the correct number for the right eye is -0.25 -0.50 x 180 while the correct number for the left eye is simply +2.75.