Centralization refers to the distance between the two temples of the frame, also known as the PD (pupillary distance) of the frame. This value is a very important parameter in the process of glasses preparation, which determines the alignment degree between the center position of the lens and the eye. The center deviation less than 2 mm is considered as the normal range. Too large or too small the center of mass deviation will lead to the misalignment between the lens center and the eye, which will cause visual discomfort or affect the visual effect.