How to Read Eyeglass Prescriptions
Lens powers are measured in diopters. The higher the number, the stronger the lens. Plus signs ( + ) are used to indicate reading glasses or magnifying lenses. Minus signs ( — ) are used for indicating distance or reducing lenses. Magnifying lenses are convex in shape. They bow out and are thicker in the middle than at the edges. Minus lenses are concave and bow in so they are narrower in the centre than at the edges. For astigmatism the lens will have an additional cylinder ground into it which is aligned to compensate for the opposite cylindrical formation of the eye.
A prescription might be written like this:
OD (right eye) —2.25 —.50 X 175 degrees OS (left eye) —2.00 —.25 X 175 degrees add OU (each eye) +1.50
This prescription is for bifocals. The right eye needs a shortsighted correction of —2.25, and an astigmatism correction of —.50 at 175 degrees. The left lens is slightly weaker, only needing a —2.00 short-sighted correction and a —.25 astigmatism correction. In addition a magnification correction of +1.50 is made on each lens for reading.
Roughly speaking, 20/100 vision needs —1.00 correction, 20/200 a —2.00 correction; however, prescriptions may be stronger to ensure that the patient has perfect vision in the worst possible lighting conditions.
Eyeglasses do not actually correct the images that your eyes see, they create distortions in proportion. Concave lenses reduce the size of the images you see, and convex lenses magnify the images. Cylinders produce some distortion even though they are aligned to compensate for the irregular shape of the eye.
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