English: Geometric figure of the projection of a point P by the pinhole camera model in order to define the image function.

An object point P is projected through the pinhole O (centre of projection) under the angle tau with the viewing axis (left side of O). Behind the pinhole the point is projected onto the image plane where it creates point P'. 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. In that situation the image function is defined such:

tan(tau) = r/c => r = c * tan(tau).

In case of a real camera the direction of the projection line changes due to lense distortion and the angles tau and tau' are not equal anymore.