The standard lens of the Pentax 645 system used to be a 75mm lens, like the SMC Pentax A 645 75mm/2.8, which can be used on the original Pentax 645, 645N and the newer 645NII bodies.To explain why 75 would be the ideal standard lens, the maths is actually easy. The dimensions of the 6 x 4.5 120 film size is simply 6 x 4.5cm, of which its name is self-explanatory, or 60 x 45mm. The diagonal length
