The perspective camera model falls nicely into the mathematical realm of projective geometry which was invented by the French mathematician, Desargues (1591-1661). This formalism has grown to contain some extremely graceful mathematics ranging from invariants to injection of affine spaces to duality theories. While it is beyond the scope of this paper to delve into this formalism, further reading can be found in [41] [45]. In the following, we shall discuss its practical implementation and implications in the SfM techniques that have adopted it.