That's a good point. The Contarex, for example, had a lever in the mirror box that when pressed raised the mirror. And this was for mounting the 21mm Biogon, whose rear element sat just in front of the shutter curtain. This lens was sold with its own viewfinder for use on the Contarex. In truth, several of the early Contarex lenses were nearly identical to those available for the Contax.
Originally Posted by Chan Tran
Personally, I've never used the mirror lockup feature on any camera. I probably would, however, if I did a lot of macro work.