Each set of prescription lenses has an "optical centre" that is determined by the pupillary distance. The pupillary distance is used to determine where you look from the lens of the glasses and should be as accurate as possible. Below are the steps to measure your own PD. Click here for more on measuring PD.


step 1. Position yourself approximately 8 inches (20 cm) away from a mirror or yourself arm’s length from your friend.


step 2. Close your left eye and align the ruler’s zero to the center of your right pupil.


step 3. Try not to move your head or the ruler.Close your right eye and note the reading directly over your left pupil. That will be your PD.


step 4. You also need to measure the distance from two pupils to the center of nose respectively to make sure that if you have two PDs (OD & OS).