Eyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Light rays are always traveling around you from the sun or various lamps. These light are constantly travel in a straight line. Why your friends have perfect eyes, while you have to wear a pair of eyeglasses all the time?

The bending of a light ray is also called refraction which is due to a change in its speed. The normal eye can refract or focus light without the help of any glasses. If the eye cannot focus an image sharply and requires another lens to assist it, then it is said that you have a refractive error. There are many types of refractive errors, and different conditions can result from them, including myopia, hyperopia, and astigmatism. » Read more: What Is Eye Correction?

Your pupil distance is the distance in millimeters between your pupils (the centers of your eyes). Your adult pupillary distance measurement never changes, and it averages 60 millimeters (mm) for women, and 64 mm for men. Sometimes your eye doctor will write your pupil distance for each eye (for example 33/34, called a monocular pupil distance). Or, the eye doctor may write the pupillary distance measurement as 67/64. This means that your pupil distance for distance vision (or DPD) is 67, and for near vision (or NPD, for reading glasses or multifocal lenses) is 64. Your near vision pupil distance is almost always 3mm less than your distance vision pupillary distance measurement.

Get your pupil distance from optometrists
The pupil distance is measured with an instrument called a “pupillometer” that is put up to your face. Any optical store employee can measure your pupil distance with this instrument; it is not required to be measured by an eye doctor. Any optical store will have a pupillometer, so eye doctors assume that you will have your pupillary distance measurement taken in the optical store. » Read more: Pupillary Distance for Your Prescription Glasses