Sensitivity of Calibration to Principal Point Position

A common practice when carrying out self-calibration of a camera from one or more views is to start with a guess at the principal point. The general belief is that inaccuracies in the estimation of the principal point do not have a significant effect on the other calibration parameters. It is the purpose of this paper to refute that belief. Indeed, it is demonstrated that the determination of the focal length of the camera is tied up very closely with the estimate of the principal point. Small changes in the estimated (sometimes merely guessed) principal point can cause very large changes in the estimated focal length. In fact, the relative uncertainty in the focal length is inversely proportional to the distance of the principal point to the epipolar line. This analysis is geometric and exact, rather than experimental.