The Evolution of the Camera
Long ago when the world was young and cameras used film, there were two kinds. With a viewfinder camera, the human looked through the viewfinder, the camera looked through the lens, and the two views were different—significantly different for close-up shots. A single lens reflex camera (SLR), typically larger and more expensive, contained an elaborate internal mechanism to permit the human to view the scene through the same lens that the camera saw it through. To take a picture, the mirror that was directing the image up to the prism that bent it to reach the human's eye swung up and out of the way, in order that the light could get to the film instead. (There is also evidence in the fossil record of a still earlier design known as a twin lens reflex).
After digital cameras came along and I started using them, it occurred to me that the image I was seeing on the view screen was the same image that would be recorded on the camera's memory—one of the advantages of using electronics instead of optics for the purpose. I was already looking through the camera's lens, so no need for a mirror and prism. Oddly enough, however, SLR's (called dSLR's since they were now digital) were still being made, were still large and expensive, still had an elaborate apparatus of moving mirror and prism, and were still regarded as what serious photographers used.
They had two significant advantages over the less expensive sorts of digital cameras—interchangeable lenses and much larger imaging sensors, permitting them to take better pictures. They also had optical viewfinders that let you look through the camera's lens. The resolution perceivable by the human eye is higher than the resolution of a camera's viewscreen, so getting the image directly to the human was worth something—but, given the quality of the available screens, not very much. That, at least, was how I saw it—making the internal mirror and related apparatus the photographic equivalent of the human appendix.
It couldn't last—and, fortunately, didn't. It eventually occurred to someone in the industry that a camera with interchangeable lenses and a large sensor but without mirror, prism, and optical viewfinder could be very nearly as useful as a dSLR but considerably smaller and less expensive.
My Sony NEX-3 arrived yesterday. It cost about half as much as a comparable dSLR—and has the same sized sensor. With the smaller of its two lenses it is not very much larger than the pocket camera it replaces.