US Patent 7936915 Focal length estimation for panoramic stitching

A focal length estimation technique is presented that estimates the focal length used to capture each of a pair of overlapping images taken by a camera rotating about its center. When it is known that the focal length is the same for both images, it is estimated using just two pairs of corresponding points between the images. When it is either unknown whether the focal lengths are the same or it is known that they vary, the focal length of each image is estimated using just three pairs of corresponding points between the images.