The film SLR was the predecessor of DSLRs. Back then, you basically had two choices - preview through a different lens than you shoot with (twin-lens reflex, ra

The film SLR was the predecessor of DSLRs. Back then, you basically had two choices - preview through a different lens than you shoot with (twin-lens reflex, rangefinder), or preview through the same lens that you shoot with (single-lens reflex). In an SLR design, you need a way to swap between the light path to the film versus the light path to the viewfinder, and a mirror and prism is the most practical way to do so. Yes, a DSLR can simply show in real time what is being captured on the sensor - this is called "live view" and was introduced some years after the first DSLR, as it's not a critical feature. But live view drains a lot of battery. It also had terribly slow and inaccurate autofocus using contrast detection, but was improved using phase detection (e.g. Canon DPAF). Another consideration is that live view takes a second or two to start up, whereas most DSLRs will let you pick up a camera from idle and shoot through the viewfinder in a fraction of a second. > any camera with exchangeable lenses These are called interchangeable-lens cameras (ILC), a category which has significant overlap with but isn't identical to DSLRs. > professional cameras with exchangeable lenses are dead? Mirrorless cameras with interchangeable lenses (MILC) are quickly becoming the new professional gear.