The Pupillary distance gets measured by opticians to position the optical center of the lenses right in front of the pupils correctly. If those optical centers are misplaced unwanted prismatic effects will be induced, requiring the wearer to turn his eyes inward, or even outward, to keep from experiencing double vision.
The wearer needs to compensate for a misplaced optical center with increased muscular strain in the eyes. Over time, this effort causes visual discomfort and can result in a decreased ability of the eyes to work together in binocular vision.
With a correctly measured pupillary distance, unwanted muscle tension gets reduced to a minimum and the wearer enjoys the best visual comfort.
As soon as a prescription is built into your glasses the optician needs to decide where to put the optical centers. The measurement of the pupillary distance is just one out of a few parameters to describe where to position the optical centers in front of the pupils of the prospect. Additional parameters are:
- Pantoscopic tilt
- distance between the lower lenses edge and the eye`s pupil
Here in the picture above you can see the red lines on the glasses being wider than the pupils. Actually, the markings on the lenses should show the cross right infant of the pupil. This PD measurement would be wrong.
What Happens If the PD Is Wrong On Glasses?
Depending on how far the optical center is decentralized or how wrong the pupillary distance is the person wearing the glasses might feel nothing up to the point where he or she experiences headaches or even double vision.
This constant strain could lead the brain to focus on one eye and switching off the other. In addition, this could result not only in discomfort but also in a compromised perception of depth. In most cases, the person still sees perfectly clear but the concentration will suffer as well as the ability to cope with difficult visual tasks like watching a movie in a 3D theater.
When the pupillary distance heavily compromises the way on how the eyes play together the induced change might not come back to the state before the PD was wrong.
What if PD Is Off by 1mm?
When the PD is wrong by a millimeter the outcome for the wearer will be very different depending on the lens power in the glasses. A wrong pupillary distance is measured in millimeters. This distance multiplied with the lens power equals the deviation a wrong PD produces. This result is displayed in cm/m. In the table below I will give you an idea of how the deviation will influence the vision of the wearer of the glasses.
A wrong PD can be calculated with the prentice rule. Every optician is familiar with that formula: P=c*s.
|0.25 Diopters||0.025 cm/m|
|1.00 Diopters||0.1 cm/m|
|3.00 Diopters||0.3 cm/m|
|7.00 Diopters||0.7 cm/m|
|10.00 Diopters||1.0 cm/m|
When the person wears the lenses and looks at an object that is one meter in front of him this object will be shifted to the side in front of one eye. When the person wearing the lenses does not look at an object of one meter but let’s say 10 meters these values get multiplied by the distance.
Such a shift of the picture can be compensated with an inward or outward rotation of the eyes. Obviously, this is not ideal Even not in a very small way. This is just irritating and as described above can result even in double vision when the shift is too big for the eyes to compensate.
Depending on the direction of where the lens is decentered and the prescription the compensation mechanisms of the eye work better or worse. When you look at eye movements it is normal for the eyes to turn inward. Every person does this when reading and the closer the book gets the more the eyes need to turn inward. So in case of a wrong PD, the displacement would be less noticeable.
But there is also the other direction. When the wrong PD produces a shift of the picture to which the eyes need to rotate outwards to make up for the wrong PD the eyes have less tolerance before problems will show up. The lens is designed to be positioned in front of the pupil to produce the best visual experience.
With a displacement of the lens, the wearer can not enjoy the full capabilities of the chosen lens design due to various optical aberrations that will minimize visual acuity.
Does Pupillary Distance Have To Be Exact?
The pupillary distance needs to be exact to avoid problems in binocular vision. Therefore the Eyecare industry constantly strives for optimized ways to measure pupillary distances. Today it is normal for opticians to measure the pupillary distance with an accuracy of one-tenth of a millimeter.
This ensures repeatable results produce a better visual performance for the person wearing the glasses and eliminates complaints effectively. To be fair depending on the person measuring the pupillary distance the measurement could suffer repeatability no matter what tools are used. It is always good to have a craftsman performing the measurements who do this daily and knows the technical pitfalls.
When the pupillary distance was not measured correctly the outcomes for the person wearing the lenses can start with no consequences at all up to an enduring change in the way both eyes play together and how depth is perceived. A wrong pupillary distance should be avoided with technical and methodological know-how to reduce the chance of a wrong PD in the glasses.
The process of measuring PD looks really simple but there are many things that could go wrong. As an optician, I can only recommend getting the measurements done by a professional. This still why after so many years people prefer glasses from optometry practices rather than online shops.
Saving a few bucks is not worth the cost of having poorer concentration, less optimal perception of depth, and possibly headaches. If you are not sure if your pupillary distance was measured correctly just visit the optical store and I am sure the eye care professionals there will be happy to help you out.
I wish you a great day.