  We state an elementary inequality for the structure from motion problem for m
cameras and n points. This structure from motion inequality relates space
dimension, camera parameter dimension, the number of cameras and number points
and global symmetry properties and provides a rigorous criterion for which
reconstruction is not possible with probability 1. Mathematically the
inequality is based on Frobenius theorem which is a geometric incarnation of
the fundamental theorem of linear algebra. The paper also provides a general
mathematical formalism for the structure from motion problem. It includes the
situation the points can move while the camera takes the pictures.
