The process of lens edging is done to make sure that blank lenses are shaped and sized to fit the preferred frame. There are a few qualities of lenses that are going to determine their overall corrective level. How Should Glasses Fit Your Eyes? The lenses of your glasses should be centered in front of your eyes so that your pupils are looking out of the middle of each lens. This rule applies both horizontally and vertically. Your eyes should not be too far up or down, left or right in relation to the lenses. Lens centering is a crucial step in the manufacture of rotationally symmetric optical systems. For optical systems to deliver maximum performance the optical design must be superior. "If you look through the lens at a distant point, the point image will not move when the lens is rotated slightly about a vertical or horizontal axis the goes through the nodal point. This is called the optical center.