In a medium format camera with removable backs, does a particular lens have limitations regarding what size film type it can handle? Is there a limit? I.e. if a camera has a 105mm lens attached and a person is shooting with a 6x6 back but then decides to shoot with a 6x9 back, does he have to change lenses? How does ALPA solve this problem (since their cameras can accept 645, 6x6, 6x9, etc )? Thanks! Jon.