Pupillary Distance (PD) is the measurement of the distance between the center of each pupil, measured in millimeters (mm). It is an essential part of ordering glasses and usually measured by Opticians. For best clarity, your PD must match the optical center of the glasses. Depending on the purpose of your glasses, either for distance, computer or reading vision, this measurement will change. Your PD is widest when looking far away and progressively narrows as you look at closer objects. Typically, adult pupillary distances range between 54 to 68 mm and children range from 41 to 55 mm.
During the day, your PD can vary about 3-4 mm depending on you viewing distance. For instance, if your PD is 61 mm when looking at distant objects (i.e. street signs, chalkboard, TV, etc.), it narrows to 57-58 mm when viewing objects within 16-24 inches from your eyes. American adults spend 9+ hours daily on digital devices that range from cell phones to dual or more monitor screens. Your PD automatically adjusts during the day, as your eyes move between different viewing distances. Fortunately, when ordering glasses including progressive designs, only the distance PD is necessary as standard adjustments are made to accommodate for reading or computer vision.
Learn how to measure your own PD at https://www.eyeque.com/newsdetail/what-is-this-funny-looking-ruler