When you go to the eye doctor for glasses or contact lenses, the doctor measures your eyes and records these readings in a prescription. Patients have a tendency to remember that they are "20/400" or "20/1000," but these numbers simply refer to the line on the eye chart that you are able to read. Doctors use a more precise set of numbers to measure your prescription.

If you look at your prescription, either for glasses or contact lenses, you can get a fairly good idea as to the type of refractive error that you have, as well as its severity. Generally, prescriptions appear as a series of three numbers and might look something like this:
 

Example Prescription

The first number (sphere) indicates the curvature of the cornea. A negative number means that you are nearsighted with a cornea that is too steep. A positive number means that you are farsighted with a cornea that is too flat. The closer the number is to zero, the less severe the prescription.
 

Range of Prescription

The second number (cylinder) indicates the amount of astigmatism you have. The further the number is from zero, the more pronounced the astigmatism. A value up to 2.50 is considered moderate; anything above 2.50 may be considered severe. The third number (axis) reflects the orientation of your astigmatism.