The real time recovery of the projection geometry is a fundamental issue in interventionnal navigation applications (e.g. guide wire reconstruction, medical augmented reality). In most works, the intrinsic parameters are supposed to be constant and the extrinsic parameters (C-arm motion) are deduced either from the orientation sensors of the C-arm or from additional sensors. However, due to the weight of the X-ray tube and the C-arm, the system is undergoing deformations which induces variations of the intrinsic parameters as a function of the C-arm orientations. In our approach, we proposed to measure the effects of the mechanical deformations onto the intrinsic parameters in a calibration procedure. Robust calibration methods exist (the gold standard is the \emph{the multi-image calibration}) but they are time consuming and too tedious to set up in a clinicalcontext. For these reasons, we developed an original and easy to use method, based on a planar calibration target, which aims at measuring with a high level of accuracy the variation of the intrinsic parameters on a vascular C-arm. The precision of the planar based method was evaluated with error propagation method and its appeared that the precision of intrinsic parameters are very good compared to the gold standard method. The method was also successfully used to assess to behavior of the C-arm with respect to the C-arm orientations. Results showed a clear variation of the principal point when the LAO/RAO orientation was changed. In contrast, the intrinsic parameters do not change during a cranio-caudal C-arm motion.