English: Geometric figure of the projection of a point P of a real camera.

An object point P is projected onto the image plane (in positive position) where it creates point P', through the centre of projection O under the angle tau with the viewing axis. Due to the fact that a real camera uses a camera lens there are two centres of projection: the middle of the entrance pupil O facing the object space, and the middle of the exit pupil O' facing the image space. The projection line travelling through the exit pupil O' has an angle to the viewing axis tau' and creates the point P' on the image plane (in negative position).

In an ideal case of the pinhole camera model the direction of the projection line remains equal and thus the angles tau and tau' are equal as well. But a real camera usually suffers from lens distortion and thus the angle tau' changes. The intersection of the optical axis with the image plane creates the point of symmetry of distortion S. The main part of distortion is radial-symmetric to this point S.