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Home > FAQ
what is pupil distance? how to measure PD?


Pupilillary Distance (PD) or interpupillary distance (IPD) is the distance (the industry standard is to measure in millimeters) between the centers of the pupils in each eye. This measurement is used when preparing to make prescription eyeglasses. Positioning lenses correctly in relation to the centre of the pupils is especially important for higher powered lenses due to the location of the optical centre  of the lenses. It can also be relevant to binoculars : they must be adjusted to suit the user's IPD; and the minimum allowed by some binoculars is still too great for people with a small IPD.

Someone with training in the field of optics can accurately measure your pupillary distance, or you can make an approximate measurement yourself using a ruler, with someone’s assistance or a mirror. The measurement will usually be taken twice:

1st with the patient focusing at a distance, and 2nd with the patient focusing at something close. The second measurement is used for reading glasses, and bifocals however the rule of thumb is that the so-called "near PD" will be 4 millimeters less than the "far PD" when the far PD is over 60mm, or 3 millimetres less than the far PD when the far PD is undre 60mm. It is recommended, due to accuracy, to measure the PD with a pupilometre , however some practitioners take preliminary measurements with a millimeter ruler placed across the bridge of the nose.

Note: Regarding the TTL loupes or TTL combine Flip Up loupes, the pupil duistacne is need IPD (near pupil distance). and the prescription lens need far Pupil distance. if you have your own far pupil distance,just need reduce 3mm (pupil distace is less than 60mm) or 4 mm (if your Pipil distace is over 60mm) , you can get your near pupil distance.  

PD is the distance between the centre of one pupil to the centre of the other pupil. Prescription glasses are made so that the distance between the optical centres of the glasses' lenses, is the same as your PD. PD varies from person to person but once you are an adult, your PD does not change. Therefore, as an adult, if you have had it measured, then it will still be the same, even if your spectacle prescription has changed. Once you know your PD, you do not need to measure it every time you buy prescription glasses.

PD is measured either during the eye test or when prescription glasses are ordered. Obviously we cannot measure your PD over the internet, so it is best to make sure your PD is included in your glasses prescription.  

If your prescription does not contain your PD:

  • Ask to have it measured at an optical shop ?some may charge a fee.
  • Contact an optical shop where you have previously purchased prescription glasses. Your PD has been measured if you have ever had prescription glasses made up.
  • Your previous optometrist may have a record of your PD.
  • Alternatively, try measuring your own Pupillary Distance as explained below. (Note: Measuring your own PD may not be as accurate as having a trained person do it. If your head or the ruler moves during the measurement then the reading will be inaccurate)

The higher the lens power in the glasses prescription, the more important it is to use an accurate PD. These procedures may not work for everyone. We recommend that you obtain a professional measurement of PD whenever possible.

Measuring your own PD (Pupillary Distance):

TIP: This should be done with glasses off, if the numbers on a ruler are difficult to see, then a magnifying mirror may help.

1. Whilst looking into a mirror hold a ruler against the bridge of the nose with one hand.

2. Close your left eye, and line the '0' up with the centre of the pupil of your right eye as shown in the diagram.

3. Without moving your head or the ruler open the left eye and close the right eye. Read the number that lines up with the centre of the pupil of the left eye. This number represents your Distance PD in millimetres. You have just used this technique to measure your distance PD.

4. Repeat this whole process at least 3 times to try and get a consistent measurement in millimetres. Make sure your head and the ruler do not move after lining up the zero on the ruler until you take the reading. This procedure may be difficult if one eye has very poor vision compared to the other eye.

how to get your near pupil distance? the near PD normall less 3mm or 4mm than PD
e.g. Distance PD = 60
        Near PD= 60 -3 = 57

Sometimes the centre of the pupil is difficult to see. Instead of lining up the ruler with the centre of the pupil, it is more precise to use the edge of the pupil as shown at the bottom of the page; (be careful to use the inside edge of one pupil and the outside edge of the other pupil.)

Getting a Friend to Measure Your PD:

This technique uses a second person to measure your Pupillary Distance (PD).

1. Both of you should be sitting down approximately 45cm apart. The person having their PD measured keeps both eyes open. When your friend is taking the measurement, they must keep one eye closed.

2. The ruler is held against the forehead as shown above. The person having the PD measured looks into the open eye of the person taking the measurement.

The ??is lined up with the centre of one pupil. Record the number lining up with the centre of the other pupil in millimetres. This is the measurement for the near PD? Neither person should move their head during this procedure. This procedure will not work if the person having their PD measured has a turned eye. You have just used this technique to measure your near PD.

if you get your near pupil distace , the far distance is add 3mm. (This rule is accurate for most people).

Eg Near PD= 57
Distance PD= 57 + 3= 60

Instead of using the centre of the pupils, you can use the edge of the pupils as shown below. (be careful to use inside edge of one pupil and outside edge of the other pupil.)


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