Two people try the same demo glasses in the showroom, one after the other. The first sees the display floating crisp and centred in front of them; the second says the image sits slightly to one side and looks clipped at a corner. Same glasses, same software, same text — the difference often comes down to a single number almost nobody knows about their own face: their pupillary distance.
Short answer: On ordinary glasses your pupillary distance can be a millimetre or two off — at worst the correction gets very slightly worse. On a display glasses like the Even G2 it matters noticeably more, because the projected image is only fully sharp within a limited region — and your pupil has to land inside it. So the PD aligns two things at once: your vision correction and the display to your eye. That is why it belongs to a professional measurement, not a phone-app guess in the mirror.
What pupillary distance actually is
Pupillary distance — PD for short — is the distance between the centres of your two pupils, measured in millimetres. For most adults it falls roughly between 54 and 74 mm, but your own value is as individual as your shoe size. It is often given monocularly, meaning per eye relative to the bridge of the nose, because almost no face is perfectly symmetrical. Any serious optician measures it before making prescription lenses — it decides where the optical centre of the lens sits, the point you ideally look through.
Why the PD suddenly counts for more with a display
On a classic pair of glasses a small PD error forgives a lot. If the optical centre sits a touch too far out, you still see well; only larger deviations create a slight pull or unwanted prismatic effect. A display behaves differently. The Even G2 projects its image into the lens, and that image is only complete and edge-sharp within a certain viewing region in front of the eye — in AR jargon, the "eyebox". If your pupil sits too far outside that region, the display can look slightly shifted, clipped at an edge, or less clear towards the margins.
Ordinary glasses
The PD centres the lens's optical zone over your pupil. A small error only degrades the correction slightly — most people never notice it day to day.
Display glasses
The PD additionally helps align the projected image to your eye. If the pupil lands beside the viewing region, it is not your acuity that suffers but the display itself — it can look clipped or unclear.
That the PD does double duty here is our professional reading of the principle, not a figure Even has published — the maker gives no public tolerance in millimetres, and we will not invent one. The direction holds regardless of the exact number, though: the better the display is aligned to your eyes, the calmer and more complete the image stands.
How the PD is handled at the fitting
This is exactly why a fitting is no formality on a display glasses. Several things happen together on site. First, your PD is measured cleanly — with a pupillometer or measuring rig, not a ruler from a drawer. Second, that value feeds into the prescription lenses for the Even G2, so the optical centre sits where your eye actually looks through. Third — and this is the part that does not exist on ordinary glasses — the fit of the frame is adjusted so the pupil sits as well as possible relative to the display.
The components the Even G2 already brings help here: two frame variants with differently wide bridges (G2 A around 22.6 mm, G2 B around 19.5 mm), adjustable nose pads and an adjustable temple width. These levers help decide how high and how centred the glasses sit on your nose — and therefore where your eye lands relative to the display. Which combination suits your face is best settled in conversation; for a first sense of whether the G2 fits your life at all, see our page Even G2 — who it's for.
Why the phone app in the mirror isn't enough here
There are apps and guides that let you estimate your own PD — card on the forehead, photo in the mirror, read it off. For a first orientation that is fine, and for a simple spare pair some people even get by with it. For glasses that place an image in front of your eye, we would not rely on it. For one thing, these self-measurements are more error-prone than they look — even a slight head tilt shifts the result. For another, the number alone is only half the story: it works together with the real fit of the frame, and no app can set that from a distance. The few minutes of measurement and fine-tuning on site are well spent.
Honest about the uncertainty: we cannot tell you "beyond X millimetres of deviation you stop seeing the image" — Even has published no such figure, and an invented one would be worthless. What we can say is that PD is among the values where care pays off, precisely because the display draws on it too. When in doubt, get it measured properly once.
So: buy, skip, or test first?
Buy with a clear conscience if you are going through a fitting with a professional PD measurement anyway — that is the normal case, and exactly the route on which the display ends up cleanly in front of your eye.
Don't order on a hunch if you were planning to use a PD roughly guessed somewhere and buy the glasses sight unseen. It can work out, but you give away the very step that helps most on a display glasses.
Testing first is especially telling here: put on a demo pair and watch whether the display sits centred, complete and edge-sharp. That is exactly what no spec-sheet number can predict — it is your face on this device. Come to the Zürich showroom for it, where your PD is measured and the frame is fine-tuned to how it sits on you.
Disclosure: AI-Eyewear is the authorised Swiss reseller of Even Realities. We would rather do a clean measurement than ship a quickly ordered pair that then doesn't sit ideally in front of your eye.