PD is quite simply the distance between your two pupils. The measurement is taken from the central point of each pupil, and is recorded in millimetres.
Adults normally have a PD in the range of 54 to 74mm, where children normally range from 43 to 58mm.
Your PD measurement is used in the alignment of your glasses to make sure your pupils are lined up with the centre of each lens. For optimal vision, it’s important that you are looking through the central part of the lens, so you need to make sure you measure pupillary distance accurately.
People sometimes talk about two different types of PD measurement: single and dual PD. Single PD is the distance between the centres of the two pupils. Dual PD is a measurement that consists of two numbers: the distance between each of the pupils independently and the bridge of the nose. The first number of the dual PD measurement is always the right eye and the second is the left eye.
If you’re struggling to accurately measure PD, there are a few things you can do to make the process easier.
You can get millimetre rulers specifically designed for PD measurements with a cut-out section in the middle to fit over the bridge of the nose. You can even print out PD rulers from the internet.
If you’re having trouble deciding where the centre of the pupil is, you can measure from the far-left edge of the iris (the coloured circle around the pupil) of one eye to the far-left edge of the iris of the other. This measurement works out the same as from the middle of one pupil to the other.
If you are looking at an object close up—for example, when you’re reading—your pupils tend to move closer together. So when you are calculating the distance between your pupils for the purposes of reading glasses (a measurement known as near PD), you need to adjust the figures.
In order to arrive at your near PD measurement, you need to subtract 3mm from the regular PD measurement (known as a distance PD). To calculate a near dual PD, subtract 1.5mm from each of the two measurements.
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