Superimposition is an efficient method for evaluation of coincidence between a skull and a photo portrait. The principle of superimposition method lies in the projection of the skull into the face image. During the projection of an object with a perspective camera, the mapping of a three-dimensional object into a two-dimensional image takes place. The acquired images of the same object are more or less distorted due to various photographic conditions, due to extrinsic and intrinsic parameters of the camera. The distortions have important influence onto reliability of human identification by the superimposition method. Mathematically we can describe most of the distortions. On the basis of the description the divergences could be simulated and in some cases eliminated by geometric transformation of the compared images. We are presenting a mathematical model of the standard projective camera and the mathematical description of distortions which are important for the superimposition process. The results show the distortions and the elimination of the distortions by means of the projection model.