Another possibility is that the secondary mirror may be sticking.
If you remove your camera lens, you'll see the reflex mirror on a 45º angle and it's job is to bounce the light up onto the focus screen (the frosted glass you see in the roof of the camera chamber... it's like a mini back-lit movie screen. When you look through the camera view-finder, that's the screen you're looking at. -- btw don't touch it with your fingers because it's a real pain to clean it without damaging it.)
Anyway... what you may not realize is that the center of that reflex mirror is actually semi-transparent and hiding behind it is a secondary reflex mirror and this mirror is bouncing light DOWN into those focus sensors on the floor of the camera.
I'm wondering if the mirror mechanism is sticking and the secondary mirror isn't properly swinging back into the correct position. It is a moving part so as the equipment is used, it's the sort of thing which is subject to wear. Shutter mechanisms are sort of like the tires on your car... with each use you hardly notice any wear... but after enough years and miles driven, you know that you will eventually need to replace them. The shutter mechanism will also eventually need to be replaced after enough years.
I would probably start with some air to see if it isn't just a dust bunny that can be blown clear. (it's cheap and easy to test and if you don't already own a hand-squeezed blower, you'd need one anyway.) But if that doesn't work, you may want to contact an authorized Canon service center.
Tim Campbell
5D III, 5D IV, 60Da
