Can one change the aspect ratio of the 5D Mark III IN THE VIEWFINDER????

[TheBlimmer]

I know how to change the aspect ratio of the 5D Mark III on the Live View Screen, but i really want to see that in the viewfinder. Is there a way??? 

[Waddizzle]

I don't think so.  The viewfinder is showing you a reflected view of through the lens.  The LCD is showing you a processed view from the image sensor. 

I would imagine that if you could find a focusing screen, made by somebody from somewhere, that showed you a cropped/framed image, then that would solve your problem of seeing different aspect ratio in the viewfinder.  But, good luck with that.

They do make hoods that you can put over the LCD that have viewfinder like eyepieces, though.