A collegue of mine asked a simple question; why does a digital SLR need a mirror? I kinda gave him some arm waving guess about not wanting the sensor to be on all the time, a friend suggested the battery draining if you put a display in front of the viewfinder to show what the sensor sees. The collegue's theory is that the manufacturers didn't want to redesign anything so they just replaced the film with a sensor and kept all the old hardware. I think it's been a long enough time that the current mirror setup must still be superior. Does anyone know the real technical advantages of the current mirror setup besides battery savings?