  Multi-view Geometry is reviewed from an Algebraic Geometry perspective and
multi-focal tensors are constructed as equivariant projections of the
Grassmannian. A connection to the principal minor assignment problem is made by
considering several flatlander cameras. The ideal of the quadrifocal variety is
computed up to degree 8 (and partially in degree 9) using the representations
of $\operatorname{GL}(3)^{\times 4}$ in the polynomial ring on the space of $3
\times 3 \times 3 \times 3$ tensors. Further representation-theoretic analysis
gives a lower bound for the number of minimal generators.
