English: Visualisation of the principles of the imaging geometry of a camera. A real camera differs in many ways from the pinhole camera model. The camera lens consists of a number of lenses and an aperture stop. The entrance pupil is the image of the aperture stop facing to the object. The exit pupil is the image of the aperture stop facing to the image plane. The centre of the entrance pupil is the centre of projection O, and the centre of the exit pupil is the point O'. The optical axis joining both points O and O' is usually not exactly perpendicular to the image plane. The intersection of the optical axis with the image plane creates the point of symmetry of distortion S. The perpendicular projection of the point O' onto the image plane creates the principal point H, and their length is the principal distance (or camera constant) c. An object point P has an angle to the viewing axis tau, and its imaged point P' has the angle tau'. Both angles are usually not equal due to distortion caused by the camera lens. All the rays originate from a point P and passing through the lens are narrowed by the aperture stop. This can be seen as a cone and thus is called ray cone. The one ray representing the ray cone which travels through the centre of projection O is called principal ray.