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Pupil Distance

Pupil Distance

You will read on the net that an average is fine, or it hardly matters etc, so what is the truth regarding the pupil distance, or PD?
Well it depends on the strength of the prescription, a small prescription with the PD slighly off will be no problem at all for most, but a large prescription a long way off will cause a certain degree of strain to the muscles which control the direction of gaze.
The eyes default to looking straight ahead, turning requires effort.
Imagine a beam of light passing through a lens and converging to a point, there would be a line down the middle of the beam where there is no deviation. When your specs are made this part of the lens (the optical centre) is placed in front of your pupil and your eyes gaze ahead comfortably. If the lenses are not centred then the image in front of your pupil is not heading in straight, so to speak, so the eye needs to adjust direction to pick it up.
You can not damage your eyes with incorrectly centred lenses, but fatigue may be experienced with headaches and possibly double vision in extreme cases. In short the larger the presription, the more accurate the PD needs to be for comfortable vision.
Varifocal lenses require an accurate PD, they have a channel of clear vision as you move from distance to near and it is important that this is correctly placed.
NOTE if you come across sites which claim to have a formula for measuring PD this is a lie, no such formula exists.

Measuring Pupil Distance

You may be lucky, your PD may be noted on your prescription. If not you will need it measuring but when you next go for your eye test maybe ask them to measure it for you for.
The distance needed is that between the pupils when the eyes are gazing into the distance. If the eyes are looking at something close (including the person carrying out the measurement) the pupils will be slightly closer together.
We recommend the following method.
  • Recruit a willing assistant.
  • Face each other, stood or sitting so both parties at a similar height.
  • The subect being measured should look beyond the measurer, maybe over their shoulder, at a distant object, or one at least a few metres away.
  • Depending on the eye colour, the easiest way to measure pupil distance will be to measure at the opposite edges of the pupil, or the iris as shown, with a millimetre rule placed against the forehead.
  • Measure a few times to make sure you are seeing consistent results.
Your measurement will normally be between between 60 and 65mm though it is not unheard of for a pupil distance to fall within 55 to 70mm.
If you are not confident you have the correct measurement you can send a pair of specs to us, and we can take the pupil distance from those.
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