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How To Read Eyeglass Or Contact Lens Prescriptions

        A glasses or contact lens prescription is written in diopters. A diopter is often written as a capital "D" but in most prescriptions it is omitted. No refractive error (glasses or contact lens prescription) is referred to as "plano or plano sph", often represented as "pl". The larger the refractive error, the larger the number for both the sphere, representing the amount of myopia or hyperopia, and the cylinder, representing the amount of astigmatism.

        O.D. is an abbreviation for the Latin oculus dexter, meaning right eye. O.S. is an abbreviation for the Latin oculus sinister, meaning left eye. The first number is the sphere. A negative number corrects for myopia. A positive number corrects for hyperopia. The second number in this prescription is the cylinder (astigmatism), and the third number is the axis of the astigmatism. The axis of the astigmatism does not relate to the amount of cylinder. It describes its orientation in degrees. If the patient has no cylinder, then the last two columns may remain blank, or "sph" (sphere) or "DS" for (diopter sphere) may be written. This prescription shows that this patient has -3.75 diopters of myopia with -0.75 diopters of astigmatism at an angle of 15 degrees in the right eye. The left eye is plano with -0.75 diopters of astigmatism at an angle of 165 degrees.
Minus Cylinder or Plus Cylinder :
        Eye prescriptions can be written in two methods, minus cylinder or plus cylinder. These give the same information and exist mainly for historical reasons. There are technical reasons which favor one over the other but it is not likely that doctors will ever agree on one writing system over the other (similar to the USA keeping its non-metric system). The main importance is to correctly write the sign of each number. The above prescription, written in plus cylinder, is Right Eye: - 4.50 + 0.75 x 105° and Left Eye: -0.75 + 0.75 x 75°
Add :
        This is the extra plus power added for near vision at the bottom of the glasses. In the past, this was usually in the form of a bifocal or a separate pair of glasses for near. Today, progressive lenses (no line) are commonly used for this purpose. In this prescription the patient required +1.50 diopters of power over the distance prescription in order to see near objects comfortably.
PD :
        P. D. is the pupillary distance (really interpupillary distance). This is often absent from the prescription from the eye provider's office. In this case, the optician who makes the glasses will take this measurement. It is important, especially in higher-powered lenses, for the comfortable alignment of the vision through the lenses. The PD can be written as the total or as the distance of each eyes pupil center to the middle of the nose bridge. The patient for whom this prescription was written had a 33 mm distance from the right pupil center to the nose bridge and a 34 mm distance from the left pupil center to the nose bridge. Thus, the total is 67 mm. When looking at a near object the total PD decreases about 3mm because the eyes turn in to see near (converge).
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