Calibrating sequences where a camera moves without zooming is often considered as a pure viewpoint estimation problem. Here, we demonstrate that considering varying principal point while keeping the focal length constant allows us to obtain a reprojection error similar to the one obtained with full calibration while imposing existing constraints on the camera (no zooming). Then, we propose a robust calibration process which can be used for any scene that contains planar structures. Robustness is obtained by estimating independently the principal point position and the viewpoint.