I don't have this 5D but another t3i and I might try the video output to some HDTV to see if this shows up there. I won't duplicate the fine manual on that but if this happened to me I suspect the mirror isn't swinging fully and given how more light can change a lens' iris diaphragm I can visualize how this could happen. Such repair is best left to Canon.
I am having a problem with my Canon 5D mark ii, I am taking some indoor food photos with off camera flashes and half of my photo (the bottom half) is black, as if it is obstructed. I don't know if my shutter release isn't opening all the way. Here is the weird part, if I go stand in my window and take a picture of the house across the street it is fine. But taking a photo indoors, half the photo is black. The exposure on the other half is perfect, with a very definitive line. I changed lenses and have the same problem with both lenses (both Canons, one 70-200mm and the other 50mm). It doesn't matter what angle I shoot at or how close I am to the subject, even if I point it at the backdrop I still get this problem. I have moved my flashes around, the entire area is well lit. I also have an overhead light, this is not a shadow. I have no idea what's wrong or how to fix it. Please help, if you have any ideas I would really appreciate it.