When: 1948.
Why: First trully modular SLR camera.
Hasselblad cameras are still used by professionals (mostly) and they still have the same modular design - interchangeable everything: lens, back, viewfinder. 1600F was the first camera by Hasselblad (excluding military aicraft models) in their long line of medium format SLRs. These cameras are somewhat expensive, but they offer excellent flexibility. These days you can even get a digital back that attaches to the same film body.
