Pupillary distance is the measurement from the center of one pupil to the center of the other. If you are getting single vision reading glasses? use your near-vision PD, which is your distance vision PD lowered by 3mm.
Why Is It Important?
Every set of the prescription lenses has an "optical center", which is determined by pupillary distance. Pupillary distance is used to determine where you look through the lens of your glasses and should be as accurate as possible.
Review and Submit
Use the example to double check your photo. Make sure you’re looking straight ahead and holding your card under your nose.