I just noticed an interesting phenomenon with my 70D. Whenever there is no battery in the camera, the optical viewfinder becomes blurry and a couple stops darker.
At first I thought the lens stops down and loses focus without power, then quickly rejected that idea because no lens works that way, and the ovf is darkened even without a lens attached.
I am now rather curious about the two questions at hand:
1. What mechanism makes the ovf lose focus in the absence of power? I was not aware there were any focusing components in the system.
2. What in the world would darken the image without power? Perhaps some reverse electrochromic layer that is necessary for the lcd overlay to work, but then why would they lighten 95% of the display with power, rather than just darken the 5%, like the standard lcd overlays?