What is PD (Pupillary Distance)?
Pupillary Distance (PD) measures the distance between the centers of your pupils. The average PD is between 54-74 mm for adults. Usually, you can find your PD number on your prescription paper after an eye exam.
Why is the PD number Important?
For prescription lenses, they have an “optical center” which depends on the PD. PD is used to determine where you look through the lens and it should be accurate if you order prescription glasses. If your PD does not match the centers of your pupils, your vision will be affected. You may feel blurry vision, headache, eye strain, and fatigue. If you order high-prescription glasses with the wrong PD, then things will be worse.
What are Single PD and Dual PD?
Single PD is the pupillary distance between the center of one pupil to the other.
Dual PD is usually written like this: 30/31. In this example, 30 is your right eye(OD) measurement, and 31 is your left eye (OS) measurement. It is the distance between the centers of each pupil to the bridge of your nose.
You can use Single PD or Dual PD to order if the number on your left and right eye are almost the same, like 30/31. But if they vary, like number 30/35, Dual PD would be more accurate.
What is Distance PD?
Distance PD is often written as “Dist PD” on the prescription paper, it means the distance between the pupils when looking at a distant object.
What is Near PD?
Near PD is the distance between the pupils of the eyes when looking at a near target. Near PD is usually used for making reading glasses.
How to Calculate Near PD if you know your Distance PD?
The difference between Distance PD and near PD is usually about 2-3mm. To get near PD for reading glasses, you only need to subtract 3 mm from your single PD(Distance PD) or subtract 1.5 mm from each eye’s number in your dual PD(Distance PD).
How to Measure Your PD?
Method 1- a Traditional PD measurement method:
- Stand about 20 cm away from a mirror with your face parallel to the mirror. Hold the ruler against your brow, keeping it exactly horizontal.
- Align the ruler’s zero (0) measurement to the center of your left pupil, then close your right eye. When your eyes look straight, open your right eye and close your left eye. Read the millimeter line with the center of the pupil of your right eye. This is your PD number.
- Repeat these steps at least 3 times to get an accurate and consistent measurement.
Method 2- for Apple users:
If you don’t know your PD number, you can measure it by APP, that a good way of PD measurement. Go to the Apple Store, search “zernike” and find an APP called “PD+”. It is a free APP that you can use to measure PD, and you can get both Distance PD and Near PD in this app.
Method 3- for Android users:
You can download an APP called “GlassesOn | Pupils & Lenses” in Google Play, or you can search “Glasseson APP” in your browser to download it. It is also a free APP, but you may need a standard magnetic card to start. You can get only Distance PD in this APP.
The most convenient and accurate method is asking your eye doctor to tell you the PD number when you have an eye exam, but if it is not included in your prescription paper, you can use these PD measurement methods to measure by yourself.
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