Among the great number of eyeglass wearers, many people can not understand an eyeglass prescription. These individuals just receive an eye exam from an eye care practitioner and also buy glasses from this channel. It is needless to even read the prescription if a patient resolves his visual needs in this simple way. But this is not the only case. Buying eyeglasses from other sources such as online stores requires customers to know items on an eyeglass prescription.
An eyeglass prescription usually provides a pair of numbers. Representing the left and right eyes, the two numbers are respectively abbreviated to OS and OD. These two numbers can be either positive or negative. A positive number indicates farsightedness while a negative one signifies nearsightedness. In detail, positive numbers have a plus mark and negative ones are prefixed with a minus mark.
In some cases, only two numbers are not enough. An eyeglass prescription for astigmatism has three numbers. Two of them are the same as a normal prescription and the third one stands for the severity of astigmatism. A larger number indicates more serious astigmatism. Furthermore, the third number is between zero and one hundred and eighty, showing the location of astigmatism. This location information is necessary for eye doctors to make proper lenses with appropriate vision correction. Each of the numbers being either whole numbers or fractional ones should be described precisely. Only accurate numbers can ensure proper vision correction for individuals.
Another number that may affect corrective properties of eyeglasses is the pupillary distance (PD), which indicates the distance between the two pupils. This number is necessary if one person contemplates ordering prescription glasses online. Some eyeglasses imprint the PD number on the frame. Without an old pair of glasses, the PD number can be easily measured using a ruler and a mirror. Remember to measure distance between the centers of the two pupils.