  Grassmann tensors arise from classical problems of scene reconstruction in
computer vision. In particular, bifocal Grassmann tensors, related to a pair of
projections from a projective space onto view-spaces of varying dimensions,
generalise the classical notion of fundamental matrices. In this paper we study
in full generality the variety of bifocal Grassmann tensors focusing on its
birational geometry. To carry out this analysis, every object of multi-view
geometry is declined both from an algebraic and geometric point of view, e.g.,
the duality between the view spaces and the space of rays is explicitly
described via polarity. Next, we deal with the moduli of bifocal Grassmann
tensors, thus showing that this variety is both birational to a suitable
homogeneous space and endowed with a dominant rational map to a Grassmannian.
