PD, which is short for pupillary distance and sometimes is also called the interpupillary distance, or IPD, refers to the distance between the centers of the pupils in each one of the eyes of human beings. When people are preparing to choose their own prescription eyeglasses, the doctor will take the measurement of their PD in order to make sure that the lenses can be positioned lenses correctly in relation to the centre of the pupils of eyes and to ensure the correct centration of your prescription lenses in front of your eyes. This measurement is especially important for people who have to wear higher powered lenses, which is because that the location of the optical centre of the lenses should be adjusted to suit the user's IPD. This just can also be relevant to binoculars. Children, adults, females and males all have different pupillary distances, and the typical pupillary distance for adults is around 54 to 68 mm, while measurements generally fall between 48 and 73 mm.