Pupillary distance: the number that decides whether your lenses are properly centred

Lenses, 5 min read, August 2026

A prescription gives the power of your lenses. It does not say where to place them. That is the job of pupillary distance, and it is where much of your comfort is decided.

A prescription lens is not a uniform plate. It has an optical centre, a precise point your gaze must pass through to get exactly the correction prescribed. Shift that point by a few millimetres and the lens still corrects, but it adds a parasitic effect along the way, rather like a slight prism. The brain compensates, often without you noticing, and that permanent compensation is tiring.

What the number actually measures

Pupillary distance, usually written PD, is the distance between the centre of the right pupil and the centre of the left pupil. In adults it most often sits between 54 and 74 millimetres. In children it changes as the face grows, which is why it has to be remeasured for every new pair.

There are two ways of noting it. As a single overall value, one number, 63 for example. As monocular values, two numbers, 31.5 and 31.5 for example, each measured from the middle of the nose. The second method is the right one, because faces are not symmetrical. A 1.5 millimetre difference between the two halves is entirely ordinary, and an overall centring would place that difference in the wrong spot.

Why bad centring is something you feel

When your gaze does not pass through the optical centre, the lens bends the light slightly. The image stays sharp, but it arrives at a very slightly different position for each eye. The eye muscles correct that gap continuously.

The classic symptoms are unspecific, which is what makes the problem hard to identify: headaches at the end of the day, a pulling sensation around the eyes, discomfort when reading, the feeling that something is off without being able to say what. Many wearers conclude their correction is wrong and go back to their ophthalmologist, when in fact the power is right and the centring is not.

The stronger the correction, the more pronounced the effect. On a low-power lens, a two millimetre offset often goes unnoticed. On a strong lens, the same offset becomes very uncomfortable. On a progressive lens it is more sensitive still, because the height, the vertical position of your gaze within the lens, also has to be placed correctly.

"A correct prescription, badly centred, gives you a pair you eventually stop wearing without ever understanding why."

Why it is not always on the prescription

Pupillary distance is a fitting measurement, not a medical one. Depending on the country and the practitioner, it may or may not appear on the prescription. Its absence is not an oversight, it simply belongs to the optician's work, and is taken when the frame is chosen.

It also depends partly on the frame you choose. The distance between lens and eye, the angle of the face, the curve of the front all change the real centring. That is why a measurement taken with the frame on your nose is more reliable than a number on its own.

Measuring it yourself, and the limits

Home methods exist: a ruler in front of a mirror, a photo with a credit card for scale, a phone app. They can give you a useful ballpark figure, with two serious caveats.

First, precision. A one millimetre error is easy to make and enough to cause discomfort on a medium or strong correction. Second, where your eyes are focused. When you measure yourself close up, your eyes naturally converge inward, which shrinks the measured distance compared with your distance-vision figure. An optician measures in distance vision, or corrects for that convergence.

If you are ordering online and have no other option, measure three times and keep the middle value. But for a correction above roughly 2 dioptres, a marked astigmatism or progressive lenses, an in-store measurement is not a nicety, it is a requirement.

What to check on collection

When you pick up a new pair, take thirty seconds. Look straight ahead into the distance and ask your optician to confirm that your pupils line up with the marks they made. Discomfort that is still there after a week of adaptation deserves a check, and should never be accepted as normal.

Get your eyes measured: our optometrists take the monocular pupillary distance during the exam, free in store, details on Free Eye Exams in Bali.

This article is informational and does not replace an eye exam. If visual discomfort persists, consult a healthcare professional.