In this paper we aim at reconstructing 3D scenes from images
with unknown focal lengths downloaded from photosharing
websites such as Flickr. First we provide a minimal
solution to finding the relative pose between a completely
calibrated camera and a camera with an unknown focal
length given six point correspondences. We show that this
problem has up to nine solutions in general and present two
efficient solvers to the problem. They are based on Gr¨obner
basis, resp. on generalized eigenvalues, computation. We
demonstrate by experiments with synthetic and real data
that both solvers are correct, fast, numerically stable and
work well even in some situations when the classical 6-point
algorithm fails, e.g. when optical axes of the cameras are
parallel or intersecting. Based on this solution we present a
new efficient method for large-scale structure from motion
from unordered data sets downloaded from the Internet. We
show that this method can be effectively used to re...