I have been borrowing a 5D Mk2 for the past week and have it for the next week too. I shot some low light events with it and loved it. I didn't notice anything on the focus screen during this event, but I really was not observing the details of the whole viewfinder, just right around the center AF point. Everything outside of the AF "circle" was pretty much dark so there wasn't anything to see anyway. Today I took the 5D outside and I noticed what appears to be a big scratch on the focus screen. Since I just noticed it today I think I caused it but I'm at a loss as for how. All I have done is change lenses a few times and I have been extremely careful because the camera is not mine. I'm not stupid enough to stick my finger (or anything else) into the mirror box and try to touch anything. The suspected scratch is about 1/3 the way up from the bottom of the viewfinder. It stretches almost the full length and in a few places is more like a scrape with some lateral area to it. It really looks like it would have to be a conscious effort to scratch it this bad. I've attached a picture of what the viewfinder looks like. What's visible is about 1/3 of the scratch, it extends to the left edge and right edge also.
I'm basically looking for someone to point me in the right direction for the course of action... Its not my camera and I would ideally like to return it in the same condition I got it so that I'm allowed to borrow it again in the future. I really don't want to ask the owner "hey, was this big mark in the viewfinder before you gave it to me?" I have a feeling they would freak out thinking the camera is ruined when its not. I'm assuming I should just buy a replacement screen and then install it.
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