Pupillary Distance (PD) - What It Is and Why It Changes

Yes, you can have your PD

Yes. Ask at any visit and we will give you your pupillary distance (PD), whether or not you are buying glasses from us. It is a measurement of you, and there is no reason to withhold it.

What we will also tell you, because it is true and useful: your PD by itself is not the whole job. It is one number among several, and all of them depend on the frame you choose and what you plan to use the glasses for. Read on for what it actually measures, or book an eye exam to have it measured with the frame you want.

What pupillary distance actually is

Pupillary distance positions the optical centre of each lens directly over your pupil. Get it right, and you are looking through the exact part of the lens ground to your prescription every time you look straight ahead. Get it wrong, and the lens still corrects your vision somewhere on its surface, just not where your eye actually is, which shows up as eye strain or blur that will not resolve.

PD is properly measured as two numbers, not one. Monocular PD records the distance from the centre of your nose bridge to each pupil, left and right, separately. Faces are not symmetric; most people’s two readings differ by a millimetre or more. A single combined number, split evenly down the middle, is an approximation applied to both eyes rather than a true reading of either one.

Why “one number” is wrong

The assumption behind “just give me my PD” is that it is one fixed horizontal distance between your pupils, mirrored evenly either side of your nose, true for as long as your face does not change. Every part of that misses something real.

• Not mirrored. Faces are not symmetric, so the distance from your nose bridge to each pupil is genuinely different for each eye, not one number halved.
• Not only horizontal. Fitting height, how high or low the optical zone sits against your pupil, depends on where the frame rests on your face; a millimetre of vertical drop is enough to move the reading zone off where your eye actually looks.
• Not frame-independent. Pantoscopic tilt, wrap, and vertex distance are part of the measurement, not adjustments made after it. Change the frame and these numbers change too.
• Not fixed once the frame is chosen. Bending a temple or swapping a nose pad moves the frame relative to your eyes the same way choosing a different frame does, which is why we remeasure after any adjustment.
• Not one value per person. Your eyes converge inward when you focus up close, so distance PD and near PD describe two genuinely different eye positions, not the same fact restated.
• Not independent of lens design. A single vision lens carries one correction across its whole surface, so a small centring error rarely matters. A progressive lens carries a narrow corridor of changing power, and a millimetre of error can put your eye outside it.
• Not independent of use case. Everyday wear, reading, a computer screen, and driving put your eyes at a different distance and angle, so a pair built for one task is not solving the same problem as a pair built for another.

Put together, PD is not a measurement of you in isolation. It is a property of the whole system, face plus frame plus fit plus lens plus purpose, and changing any one input changes the answer.

The frame is part of the measurement

Three properties of the frame, beyond simple pupil position, are part of a proper fitting measurement.

Pantoscopic tilt is how far the lens plane tips forward at the bottom. Tilt it more and the point where your line of sight crosses the lens moves as you look down at a book or a phone, so the optical zone has to be positioned with the tilt already accounted for, not as if the lens sat flat in front of your eye.

Wrap, also called face-form tilt, is the horizontal curve of the frame around your face. A wrapped frame curves the lens surface away from your eye at the edges, shifting where the true optical centre falls compared with a flat lens; a more curved frame needs more compensation than a flatter one.

Vertex distance is how far the back surface of the lens sits from your eye. Move the lens closer or farther and the effective power of your prescription changes with it, small at low prescriptions and meaningful at higher ones.

Change the frame’s curve, size, or fit, and all three move, which is why we measure with the frame you have chosen sitting on your face, and remeasure after any adjustment.

What the glasses are for changes the answer

Where you are looking changes where your pupils are. Your eyes converge, turning slightly inward, to focus on something close, compared with looking into the distance. That is why distance PD and near PD differ rather than being the same number reused: near PD is typically a few millimetres smaller than distance PD, with intermediate, computer-distance, PD sitting between the two. Our fitting system captures near PD as its own measurement point, not derived by subtracting a fixed amount from distance PD, a direct demonstration that “one PD” was never the right description.

A screen-heavy day changes the answer further: someone reading occasionally and someone at a monitor most of the day converge to a similar near point, but the frame often needs to sit differently to support hours there rather than minutes.

Lens design changes how much any of this matters. A single vision lens is ground to one correction across its whole surface, so a small centring error is forgiving; you may never notice it. A progressive lens has zones, distance at the top, reading at the bottom, and a corridor of increasing power between them, flanked by intentional blur. That corridor is only a few millimetres wide in places, and it has to land precisely relative to where your eye looks, or you spend your day hunting for a channel that is not quite where your eyes expect it.

Why your prescription is also an estimate

Underneath the fitting sits a separate source of estimate: the prescription itself. Your visual system is continuous and adaptive, not identical from one day to the next; hydration, fatigue, and screen time can all shift it slightly. A refraction converts that continuously varying system into a prescription written in fixed quarter-dioptre steps, because that is the resolution lenses are manufactured to.

“Better one, or two?” is a subjective judgment made by a specific person, on a specific day, not a reading off a ruler. The result is a best estimate inside a tolerance band, not a physical constant true forever. This is exactly why we welcome you back if a new pair is not working, and why a small shift between visits is the normal behaviour of an estimate being rechecked.

Why this is judgment, not a ruler

Resolving all of this, a frame’s tilt and curve, two eyes that are not quite symmetric, a distance number and a near number that differ, against how a person actually lives, is a judgment task, not a lookup.

Our staff opticians take these measurements every working day, across a wide range of faces, frames, and prescriptions. Where a number looks right on paper but does not match how someone actually holds their head or uses their screen, a second look before the order goes out is part of the job.

How we measure it

We measure position of wear, including PD and fitting height, using Optikam, an iPad-based system that captures these with the frame already on your face. Nothing clips onto the frame or attaches to it for the scan. The iPad’s own depth sensor (LiDAR) captures the position of the frame directly, from exactly how it is sitting, so the numbers describe the frame as actually worn, not a proxy measurement taken with something clipped on that could itself nudge the fit.

The system also captures near PD as a separate measurement rather than a calculation from the distance number, the same point this page has been making, stated by the equipment itself: there is no single PD to look up, only a measurement taken for the task the glasses are for.

If we adjust the frame afterward, we remeasure. And if a new pair is not working, come back in. We check the fitting measurements, adjust the frame, and work out whether a lens design change is needed.

Related

• Buying glasses after your eye exam
• Progressives vs readers
• Understanding your eye prescription
• How to get a copy of your prescription