Prior to SLR cameras, we had viewfinder cameras, and twin lens reflex, and cameras with ground glass backs. The viewfinder cameras gave the photographer problems framing the image, and focusing the image.


The twin lens reflex cameras solved that to a great extent, but imposed other limitations, like the requirement for two identical lenses.


Cameras with ground glass backs were difficult to view in bright light, but, you could compose and focus accurately. With the larger film formats in use, results are still stunning when viewed 100 years later.


The SLR provided a better solution for smaller format film, but it required a mirror. There were variations on the theme, pellix mirrors that did not move, etc, but the advantages stayed the same.


Electronic viewfinders with the ability to display the actual image from the sensor have a lot of promise, but so far, the resolution is too low. Its just a matter of time until the point is reached where photographers decide the technology is ready for the big time.