Anyone who has bought glasses online will probably have encountered the letters PD.
Pupillary distance.
It is often presented as the essential measurement needed to position your lenses correctly, sometimes with an accompanying tutorial involving a ruler, a mirror and the sort of facial concentration normally reserved for defusing explosives.
PD is important, but it is not the whole job. In fact, when I dispense glasses, I do not simply take somebody’s PD and send the number away. I measure where each eye sits within the frame they have chosen, after that frame has been properly fitted to their face.
That may sound like the same thing with extra steps, but it is not.
Pupillary distance describes the distance between the pupils. Your finished glasses need something more useful: the exact position of each eye within each lens.
Those positions depend on the frame.
A person can enter the correct prescription, measure their PD perfectly and still receive glasses that do not feel quite right. The vision may seem slightly strained, the lenses may feel peculiar when walking around or the wearer may find themselves turning and tilting their head to locate the clearest area.
Nothing on the order necessarily looks wrong. The prescription matches. The PD matches. The glasses have been manufactured exactly as requested.
The problem is that eyes do not wear glasses. People do, and people insist on having faces.
The same pair of eyes will sit in a completely different place within a small round frame than they will in a large rectangular one. One frame might sit high on the bridge while another rests further down the nose. The lenses may sit close to the eyes or surprisingly far away. The front may be nearly flat or curve around the face, while the angle at which it tilts can change how the wearer looks through the lenses.
The person has not changed and the prescription has not changed, but the relationship between the eyes and the lenses has.
That relationship is what matters.
The numbers printed inside the side of a frame are not much help here. They describe parts of the frame itself, usually the lens width, distance between the lenses and temple length. They do not tell us where somebody’s pupils will sit within it, how the bridge fits their nose or whether the frame will remain in the position where it was measured.
Two frames can have identical numbers printed inside them and fit like entirely different objects. One may sit beautifully while the other slides down the nose, rests against the cheeks and points one lens towards Coventry.
This is why the frame should be adjusted before the final lens measurements are taken. If it is measured first and fitted afterwards, altering the frame can move the lenses away from the positions used to calculate them.
With a simple prescription and a small change in position, the wearer may notice very little. Human vision is often wonderfully forgiving.
As prescriptions become stronger, the consequences become more noticeable. Looking away from the intended optical centre of a lens can introduce unwanted prism, which means the eyes have to compensate for an effect that was never part of the prescription. Some people tolerate this without complaint, while others know within minutes that something feels wrong, even if they cannot explain why.
The vertical position matters too. This is where a single PD measurement becomes particularly limited, because it tells us nothing about how high each eye sits within the frame.
For straightforward single-vision glasses, the optical centres can be positioned according to the wearer, the frame and what the glasses are intended to do. For varifocals and occupational lenses, the fitting heights become essential because the useful areas of the lens have to begin in the correct place.
Set a varifocal too low and the wearer may have to lift their chin whenever they want to read. Set it too high and they can begin dropping into intermediate or near power while trying to look further away. If the horizontal position is also wrong, finding clear vision becomes a prolonged search rather than something that happens naturally.
This is often described as a problem with adaptation.
Occasionally, it is actually a problem with geography.
The clear part of the lens exists. It simply is not where the wearer is looking.
Modern lenses allow us to go much further than basic horizontal and vertical positioning. More individualised designs can account for how far the lens sits from the eye, the angle of the frame front, the amount the frame curves around the face and the exact position in which it will be worn.
These are sometimes treated as microscopic details invented to make an optician’s measuring equipment look expensive, but changing the position of a lens changes the way its prescription behaves.
A lens sitting further from the eye does not always produce exactly the same effective power as one sitting closer. Tilting or wrapping a lens changes the angle at which the wearer looks through it. With some prescriptions these effects are small, while with others they can be significant enough to alter how the lens should be calculated. Try it for yourself by moving your glasses around, different distance and angles will make you see the world very differently!
This is one of the reasons we use Rodenstock as our main lens partner at Doctor Oculus. Their more advanced lenses give us the opportunity to account for these individual measurements and optimise the finished design around the person, the frame and its wearing position.
The opportunity is the important part.
Buying an individually calculated lens and then supplying only generic information rather defeats the purpose. The lens manufacturer can perform some remarkable calculations, but it cannot lean out of the laboratory, look at your face and discover that the frame sits three millimetres lower than everyone assumed.
Technology still needs somebody to give it accurate information.
This is not purely an argument against buying glasses online. Plenty of people order simple prescriptions online and receive glasses they are perfectly happy with. A modest prescription in a stable, sensibly sized frame can be quite tolerant of small differences.
Equally, buying glasses from a physical optician does not guarantee that every useful measurement has been taken. Somebody can spend a substantial amount on an advanced lens and still receive a fairly generic setup if the dispensing has been treated as little more than the administrative bit between choosing a frame and taking payment. I see this with my new customers all too regularly.
The strange thing is that people often concentrate heavily on the prescription because it looks medical and precise, while barely considering the measurements used to place that prescription in front of their eyes.
A prescription can be copied perfectly into a lens and still be positioned badly.
When that happens, the symptoms are not always dramatic. The wearer may simply remove their glasses more often than they did before. Their eyes may feel tired after working at a screen. Reading may be possible but oddly laborious. They may never settle into their new varifocals and conclude that they are one of those people who cannot wear them.
The glasses can be technically correct while still failing to work as well as they should.
This is why I see the prescription as the start of making glasses rather than the finished instruction.
At Doctor Oculus, the frame is fitted before the measurements are completed. We look at where each eye sits within it, how the frame rests on the nose, how close the lenses will be to the eyes and whether the position is likely to remain stable once the person leaves the practice.
We also need to know what the glasses are meant to do. Someone using a pair mainly for driving does not necessarily need the same setup as somebody spending the day at a desk, reading music, painting miniatures or working at distances that exist in no standard optical questionnaire.
This matters just as much when somebody brings us a prescription from elsewhere. Our Bring Your Own Prescription service is not simply a matter of copying the numbers into whichever frame they choose. The eye examination has already established the prescription. Our part is turning it into a pair of glasses that fits the person, suits their life and places the lenses where their eyes will actually use them.
PD remains useful information. It just cannot describe an entire pair of glasses by itself.
The prescription tells us what the lenses need to do. The measurements tell us where and how they need to do it.
Getting both right is how a collection of numbers becomes a pair of glasses that feels completely natural.