The center of the far distance part of the lenses should be centered in front of the eye. Accordingly the distance (PH) from the lower part of the lens to this center is used for fitting the lenses when the optician make the actual glasses. For the Panto Glasses. Where all lenses have the well defined classic shape of panto, we have the average PH that is used when actual measuring can not be done. Based on the Virtual Try-on photos the PH can be further calibrated. The PD is very important for the fitting of progressive glasses though.