Interchangeable viewfinders for a medium format camera like Hasselblad is a challenge to build. 35 mm cameras mostly contain the metering circuitry except Nikon F2AS, but for a box-like camera the viewfinder has to be inside the viewfinder. Rollei SLX broke the rule, and later Hasselblad models like F2A followed this scheme. In addittion to the light meter dilemma, the size of the prism created weight problem for the camera. The magnifying hood is the lightest viewfinder Hasselblad ever offered. It's other viewfinders weigh as much as the camera. Without prism, medium format cameras don't show a big enough image through the viewfinder for focusing. Hasselblad introduced the HC4, and HC70 finders that utilized a microscope type erecting optics to provide a great looking image. Pentax 645 also utilized this design.
