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Pupillary distance is the distance between the centers of the pupils of the eyes. This value is typically given in millimeters because they are a precise system of measurement which are useful for very small measurements. The pupillary distance becomes especially relevant for eyeglasses prescriptions, when it is used to ensure that glasses are properly fitted to the face of the wearer. Sometimes it is written directly on prescriptions and in other cases it may be left off, for reasons which will be explained shortly.
Optometrists and other eyecare professionals can measure pupillary distance using a device which is customized for the task. Commonly, two measurements are taken, one while the patient is focusing at medium to long range targets, and the other while the patient is looking at a close up target. The pupillary distance will be given in two values, one for distance vision and one for reading vision. This is done for bifocals or for prescriptions for reading glasses.
It is also possible to measure pupillary distance at home with the assistance of a ruler. It is usually helpful to have a friend or partner perform the actual measurement, although people can reasonably accurate when looking into a mirror and using a ruler. Pupillary distance can vary widely from person to person, and it is wise to take at least two measurements to confirm that the measurement is indeed correct, rather than assuming that it probably is on the first try.
Some optometrists leave pupillary distance measurements off prescriptions unless they are specifically asked to add them. This can be the result of forgetfulness, but more commonly it is done to encourage patients to order eyeglasses through the optometrist. Sales of lenses and frames can be an important part of the business and asking patients to order through the office also provides the doctor with a chance to check up on patients, to follow up with a fitting to confirm that the glasses fit, and to remind patients when it is time to get another eye exam.
Patients can ask that the pupillary distance measurement be recorded so that they can take the prescription elsewhere. Sometimes it is written as a single number and sometimes it is written as two. When two numbers like 33/27 are shown, it reflects a measurement from the center of each pupil to the bridge of the nose. Some people are surprised to learn that asymmetry is not that uncommon when it comes to pupillary distance.
Glasses of the wrong size will not feel comfortable to wear, and can cause vision problems. At the very least the patient may find that the vision is not properly corrected and patients can also develop headaches and eye strain because their eyes have trouble focusing through the lenses. Thus, it's important to make sure that pupillary distance is considered when fitting glasses.