1. A computer-implemented process for estimating the focal length used to capture each of a pair of images depicting part of the same scene in a case where it is known that the focal length is the same for both images, all intrinsic camera parameters except the focal length associated with a camera used to capture each image are known, and a rotation between the two images was about the camera center, said process comprising using a computer to perform the following process actions: identifying the locations of only two pairs of corresponding points between the images, wherein each pair of the corresponding points represents the same point in the scene as depicted in the two images; establishing a polynomial equation which defines a relationship between the focal length associated with the two images and a positional correspondence of the identified two pairs of corresponding points; and solving the polynomial equation to determine the value of a shared focal length associated with the pair of images, wherein the use of just two pairs of corresponding points represents the minimal number of point pairs needed to solve the equation.