An eyeglasses prescription gives measurements on how to shape the lenses.eyeglasses on notebook image by Alexander Maksimov from Fotolia.comAfter a visit to the eye doctor, the receptionist hands you a printout of your eyeglass prescription to use when ordering a new pair of glasses. You see a chart with a bunch of positive and negative signs, numbers and decimals. It might look like Latin to you, and you wonder what it all could possibly mean, or how anyone could interpret those numbers to create glasses that help you see perfectly.
1 Identify which eye the prescription is referring to. Sometimes the doctors use “R” to mean right and “L” to mean left. Other times, “OS” stands for oculus sinister, or the left eye, and “OD” stands for oculus dextrus, or the right eye. “OU” indicates a measurement for both eyes.
2 “Sphere” measurements represent the lens needed to correct your eyesight to 20/20. The lens measurements change in 0.25 increments called diopters. A positive number means you have farsightedness and a negative number indicates nearsightedness. The greater the absolute value of the number is, the worse vision you have.
3 Cylinder, or “Cyl,” indicates measurements needed in the lens to correct astigmatism or a de-focusing of the eye due to abnormal optical surface shape. The “Axis” measurement serves as a reference point for the cylindrical measurement, measured in degrees up to 180.
4 “Prism” measurements help correct problems like double vision or a lazy eye. A prism measurement is in diopters and moves an image left, right, up or down. The “Base” gives a reference as to what direction the image must be moved to correct the vision problem. The base measurements are up, down, in or out.
5 “Add” measurements denote the difference between a distance prescription and a reading glasses prescription. For example, if a distance prescription was +1 diopters, the reading prescription would be +2 diopters if it took one full additional diopter to create the reading prescription.