Dear SLR experts:
The placement of a pentaprism together with a reflex mirror in an SLR allows through-the-lens viewing.
However, why not have two mirrors? i.e., replace the pentaprism with another mirror that is fixed at such an angle that it reflects light from the lower reflex mirror into the viewfinder. (I guess this arrangement would be maciam the submarine's periscope that we all learn in secondary school.) In this way, TTL viewing is still preserved, and the lower reflex mirror can still be tilted up at the shutter release. So, please let me know why a pentaprism is needed??