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If you buy non prescription glasses, all you have to worry about are the frame measurements. But if you purchase prescription reading glasses, those numbers may seem like high school algebra: +2.00 -2.00 x 90 or +2.00 DS X 90. Here's a simple prescription glasses primer:
1) +2.00 shows the diopter with the spherical refraction error, in other words, whether you're nearsighted or farsighted. A plus means you're farsighted.
2) -2.00 shows the astigmatism. If, as in the second example, you see the letters DS or SPH, you don't have an astigmatism for the prescription glasses to correct.
3) 90 shows the angle of the astigmatism around the eye, which is shaped like a football thanks to the astigmatism. The angle can range from 90 to 180.
You may even see an addition number, such as “Add Power +1.50.” This indicates the addition of a bifocal lens.
So now that you know, you can refill prescription glasses online and go to the eye doctor with ease.