So far, we have worked with whatever image came straight out of our webcam, without questioning the way in which it was taken. However, every camera lens has unique parameters, such as focal length, principal point, and lens distortion. What happens behind the covers when a camera takes a picture, is that; light falls through a lens, followed by an aperture, before falling on the surface of a light sensor. This process can be approximated with the pinhole camera model. The process of estimating the parameters of a real-world lens such that it would fit the pinhole camera model is called camera calibration (or camera resectioning, and it should not be confused with photometric camera calibration).
The pinhole camera model