All about PD (Pupillary Distance)

Pupillary Distance or PD is the distance between the central region of each pupil of the eyes. The value of pupillary distance is necessary for the preparation of prescription glasses, as this value informs the lab where to place the optical center on each lens. Hence, pupillary distance is the basic optical measurement for prescription eyeglasses. If the pupillary distance is not measured correctly, then an undesirable prism is induced which leads to eyestrain. PD does not change in adults. If you have your PD from your previous prescription, you can always use it when you get a new prescription.

The optician takes your pupillary distance when you go for a regular eye check up and need prescription eyeglasses. However, if you’re opting for online purchase of prescription eyeglasses, you can either ask your ophthalmologist or eye doctor to measure your pupillary distance and provide you the result, or you can measure your own pupillary distance in front of the mirror through a precise, step-by-step process. This can save you the cost of visiting an optician and the results are very accurate if you follow the procedure correctly.

Here’s the procedure to measure your own PD

  • Get ready with a pencil, a straight ruler, and a mirror
  • Face the mirror and place the straight ruler right on the bridge of the nose, make sure to bring the starting point of the ruler directly beneath the center of the pupil of one eye.
  • Hold your ruler while staring straight into the mirror, and mark the position on the ruler of the pupil of the other eye
  • Now measure in millimeters the distance between these two marks. This distance is your pupillary distance or PD.
  • Repeat the procedure at least twice to make sure that you have an authentic and correct measurement.

You can also ask someone else (a friend or a family member) to measure your pupillary distance by holding the ruler. This will make your own job a lot easier i.e. to simply stand still. Always remember, whether you’re measuring your own PD or someone else is doing it for you, the ruler should be held straight and parallel to the floor.

There’s another manual method which requires you to wear eyeglasses. The procedure is:

  • Wear your eyeglasses or any eyeglass if you don’t possess your own.
  • Focus on a single far-off object. The object must be at least 20 feet away.
  • Put a dot on your right lens with a marker directly over the far-off object.
  • Repeat the procedure for the left eye.
  • The two dots will overlap into one dot over the far-off object, if the procedure is performed correctly. If that doesn’t happen, keep repeating the process until a single dot is formed.
  • Now measure the distance in millimeter between these two dots on your eyeglass lens with the help of a ruler.
  • The measured distance is your PD.
  • If you require a near pupillary distance for the purpose of computer eyeglasses or reading, follow the same process but focus on a nearby object i.e. either a computer screen or a book.