  The real projective plane has three well know isomorphic constructions: the
extended euclidean plane, unit (hemi)sphere, and three dimensional vector space
over the reals. In this paper we find the isomorphisms that map between these
three constructions. Additionally we investigate their relationship to
direction-sensitive photosensors which use lens(es) to transform light's
direction to a position on a local plane. This transformation, done by lenses
is a physical version of an isomorphism between projective plane constructions
