Normally whenever your eye isn't covering the viewfinder, you'd cover the viewfinder to prevent light leaks through the pentaprism. Canon usually supplies a little rubber cap that you attach to the Canon neck-strap. You remove the eyecup and slide the cap on in it's place (squeeze the sides where the eyecup has the rubber grip ridges and slide up -- on some of my bodies you don't have to squeeze, it'll just slide up -- but I don't own a 6D).
But I was reading the comments on your facebook post -- if I understood correctly, you said the following:
- you get the problem with three different lenses -- so it's unlikely to be a lens issue
- you tried to completely cover the camera to block any light - it is probably not leaking through the viewfinder (btw, even if it were leaking through the viewfinder, that's not a defect... all SLR cameras can do that and that's why they give you the viewfinder cover.)
You also showed a photo of the inside of the camera... you mentioned something "hanging down" but unfortunately the shot was blurry so I couldn't see what was "hanging down".
I can tell you it's almost certainly NOT the shutter. The camera has a focal plane shutter with two "curtains" which move vertically. If a shutter was sticking you'd notice the shot was unevenly lit being extra dark either at the very top or the very bottom of the image (depending on which "curtain" is having a problem) -- but your issue is a bright spot about 3/4 of the way down the right side and spikes in.
It could be something "hanging loose" and catching a reflection that on that spot.
Inside the camera there is a user-replaceable focusing screen above the mirror. When you use the viewfinder, the mirror is in the "down" position. In that position, the light is bounced up and onto the focusing screen (you can think of this as a rear-projection movie screen. When you look through the camera viewfinder, you are actually looking at the focusing screen.
The focusing screen is on a frame or tray which is hinged at the back. Normally that hinge never moves (the only time it would move is when you open it to swap the focusing screen.) If that tray is not completely closed, then it's possible the mirror isn't fully swinging clear (it would hit the tray that holds the screen in place.)
Before going any further, it is easy to scratch the focusing screen and since it's a textured surface (like frosted glass) it is important that you NOT touch it. Canon actually provides a tiny tool when you buy a replacement screen so that you can open and close the tray as well swap screens -- all without ever touching the screen (it looks like a pair of tweezers with a hook on the end.) Canon has replacement screens with various options and patterns as well as replacement original screens should you manage to damage yours (they cost about $30).
It may make more sense to watch a video: https://www.youtube.com/watch?v=YNwOr3g9gzA
I used this video because it's for a 6D specifically. But the process is the same on many other models with replaceable focus screens -- so if you're not seeing it in enough detail, just search for other videos on how to swap a Canon focus screen. While the screens are not identical between models, the basic concept of that frame that holds the screen is the same.
BTW... he doesn't get into showing how to swap the screen until about 4 minutes into that video in case you want to fast-forward.
I have heard of cases where the focusing screen tray was not completely closed and it caused a bit of a problem. The good news is that it's a very simple remedy to just close the door. It snaps shut. The tab at top center is the release for the door. Normally to open the door you gently tug that tab toward you (you pull out on it gently) and it releases the tray so that it can swing down. Again... do avoid actually touching the screen with your fingers. In the absense of the correct tool, a _very_ clean lint-free cloth might be used. Just push the tray (don't press on the screen... just the edge where you see the tab) up until you hear it click in.
Tim Campbell
5D III, 5D IV, 60Da
