  The rational camera model recently introduced in [19] provides a general
methodology for studying abstract nonlinear imaging systems and their
multi-view geometry. This paper builds on this framework to study "physical
realizations" of rational cameras. More precisely, we give an explicit account
of the mapping between between physical visual rays and image points (missing
in the original description), which allows us to give simple analytical
expressions for direct and inverse projections. We also consider "primitive"
camera models, that are orbits under the action of various projective
transformations, and lead to a general notion of intrinsic parameters. The
methodology is general, but it is illustrated concretely by an in-depth study
of two-slit cameras, that we model using pairs of linear projections. This
simple analytical form allows us to describe models for the corresponding
primitive cameras, to introduce intrinsic parameters with a clear geometric
meaning, and to define an epipolar tensor characterizing two-view
correspondences. In turn, this leads to new algorithms for structure from
motion and self-calibration.
