What is the center of the image?

A taxonomy that includes 15 different definitions of image center is presented. It is concluded that the accuracy of the image center can be an important factor in the accuracy of the overall camera calibration, and that the large variation in the position of the image center across different definitions and different lens settings makes the calibration problem complex. With proper modeling and by using appropriate definitions for all image centers in a system, the accuracy of the camera calibration can be improved.<<ETX>>