How to read your glasses prescription
When you review your prescription for glasses, you will see the numbers listed in the OS and OD headers. They are Latin abbreviations; OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye. Occasionally you will see a notation for OU, which means something that involves both eyes. In general, the further away your prescription number is, the worse your vision will be and the more vision correction you will need. A plus sign in front of the number means that you are proactive and a minus sign means that you are nearsighted. These numbers represent diopters, the unit used to measure the correction or focusing power of the lens that your eye requires. Diopters are often abbreviated "D".
For example, if your prescription says -1.00, you have a myopia diopter. This is a fairly mild amount of myopia. If you have -4.25, that means you have 4 and 1/4 diopters of myopia. This is more myopic than -1.00, and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of forward-looking and +5 would be more.
For people who have astigmatism, there will be three numbers in your prescription. The general way to write these numbers is S x C x Axis.
The S refers to the "spherical" part of the recipe, which is the degree of myopia or hyperopia that was analyzed previously.
The C refers to the "cylinder" or astigmatism, and can be a negative or positive number. Measure in diopters the degree of astigmatism you have. The higher this number, the more astigmatism you have.
The Axis is a number anywhere between 0 and 180 degrees. Reveals the orientation of astigmatism. It is not enough to specify how much astigmatism there is; You have to know where the curvature difference is taking place.
Here are two examples of how recipes for astigmatism eyes could be:
-2.00 +1.50 x 180
+3.50 +3.00 x 45
The first prescription means that the person has 2 diopters of myopia with 1.5 diopters of astigmatism and an axis of 180 degrees.
The second prescription means that the person has 3.5 diopters of hyperopia, 3 diopters of astigmatism and a 45-degree axis.
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