Pupillary Distance (PD) Test

What it measures

This check estimates your pupillary distance (PD) — the gap, in millimetres, between the centres of your two pupils. PD is the single measurement an optician or online glasses retailer needs to centre each lens correctly in front of your eye.

How it works

You first size an on-screen rectangle to a real bank or credit card. Every card is exactly 85.6 mm wide, so once it matches we know how many pixels make up a millimetre on your display. You then hold the screen against your brow and look at your reflection in a mirror (or have someone help) and drag a slider until two marker lines sit over the centre of each pupil. The separation between the markers, converted from pixels back into millimetres, is your estimated PD.

Tips for an accurate result

• 1Get a friend to help — It is much easier and more accurate if someone else holds the screen level and lines the markers up with your pupils while you look ahead.
• 2Measure a few times and average — Take three readings and average them. If they vary by more than a couple of millimetres, recheck your card calibration.
• 3Reading glasses use a smaller near-PD — When you focus on something close your eyes converge, so your near-PD for reading is usually about 3–4 mm less than your distance-PD.