PUPILLARY DISTANCE (PD) measures the distance between the centers of your pupils. This is known as an "Optical Center". This measurement is used to determine where you look through the lens of your glasses and should be as accurate as possible.
When your diopter is very low, say less than -1.50, or you buy off-the-shelf reading glasses at the supermarket, then the interpupillary distance is not very important, because the tolerance is very large, usually 3-5mm. However, if you provide the pupillary distance, or even the pupillary height, it will be very helpful when the lens mechanic cuts the lenses for your glasses. Especially reading glasses. Unfortunately, 99% of doctors don't write IPD on your prescription (one reason is that half of them don't need this data, and the second reason maybe doctors only provide IPD for patients who decide to buy glasses), here There are two options, which don't require you to go to the doctor, and can be done at home. First, search for a pupil distance measurement app on your mobile phone, such as "Eye Measure", and then you will easily get your pupil distance and face width and other data, you can directly choose the pupil distance when you purchase in our Can you email us your screenshot, and we will help you choose, or revise your choice.
The second option ：Find a family or friend to help you, put the ruler on your nose, take photos at 30 cm, we can know your pupil distance and determine whether the size of a certain glasses is suitable,the arms of glasses are too long or too short, so if you are willing to provide a high probability to avoid this problem.
The pictures should like these:
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