Odd observation 5dMkIII

Does anyone know why, when you look though your viewfinder without a battery in the camera no matter what you do the viewfinder is dark and blurred??

Try it....Open the battery door, look though viewfinder, close battery door and see the image change.

Is this something everyone knows about? I assumed the viewfinder is nothing more than a window through the lens?

It's all about the new focusing screen, the electronic one . The focusing screen stacks over the transmissive LCD screen, some people complain about this because is not interchangeable.

http://www.learn.usa.c​anon.com …_viewfinder_dis​play.htmlp

"The LCD overlay
Like the EOS 7D, both the EOS-1D X and EOS 5D Mark III use a transparent layer above the focus screen that can display black LCD markings as well as briefly illuminate these markings in red. Like many LCD panels, the beauty of this viewfinder display is that only the information that needs to show is visible and the finder can remain uncluttered for maximum ease of viewing the subject.

The LCD overlay does require a tiny amount of electrical power to operate. This is obviously no concern when the camera is turned on, but if the battery is removed the transmissive LCD suddenly loses a lot of brightness and contrast. This is perfectly normal and will return to full brightness once a battery is reinstalled in the camera (the camera doesn't have to be turned on; it only requires a functioning battery pack to draw power for proper viewfinder operation)."

learn something new everyday, cheers both for the info

At least I now know my lenses are working fine

I remember freaking out a bit when I first got my 7D. While the batteries were charging I wanted to have a look at the new bright/big viewfinder. To say I was disappointed would be a massive understatement.

Luckily, things "clear up" when you power up.