The "black square" is your reflex mirror.
Here's a video to help you see what happens inside your camera when you take a shot (or at least what would happen if you had the right lens on your camera). You'll see the slow-motion capture of the action fairly early on in the video (you only need to watch about the first 1.5 minutes worth of this. Watch the rest if you are interested.
But there's something else happening to your camera.
The camera in this video was a Canon 7D. This camera has a smaller "APS-C" size sensor -- much smaller then the sensor on your full-frame 6D. Since your sensor is larger, it's reflex mirror is ALSO larger. This means it needs more clearance to swing up and clear of the sensor when you take a shot.
But wait... there's more!
Your camera's larger sensor measures 36mm wide by 24mm tall. It has a corner-to-corner diagonal measurement of nearly 44mm.
Canon's "EF-S" lenses are specifically designed to work ONLY on Canon EOS cameras that have APS-C size sensors (like the 7D, 7D II, 80D, 70D, 60D, 50D, etc. and all of the "Rebel" models. This smaller sensor measures roughly 22mm wide by about 15mm tall -- or a corner-to-corner diagonal measure of about 28mm.
You'll notice these lenses are all "round" and not rectangular. The image they are projecting into the camera body is actually a circular image. The sensor is merely in the middle of the circle -- so part of the image is spilling off the sides of the sensor and you end up with rectangular image. The image circle simple needs to be large enough to at least cover the size of the sensor.
The "EF" lenses project an image circle into a camera body which is more than 44mm across (large enough to cover the area of the 36x24mm sensor in your camera).
The "EF-S" lenses project an image circle which is just a little more than 28mm across (only large enough to cover the area of the smaller APS-C size sensor.)
That smaller image circle is not adequate for your larger sensor camera.
And there's STILL MORE!
It turns out one difference Canon can use with EF-S lenses is... due to it's smaller reflex mirror, the rear-most lens element on the lens actually protrudes INTO THE CAMERA BODY. This does NOT happen with full-frame "EF" lenses. They can get away with this on EF-S lenses because the smaller mirror still has room to swing clear when you take a shot. But when you use a full-frame camera, that larger mirror... will actually HIT the back of the lens because there isn't enoguh space.
The mirror is trying to swing up... smacks into the back of your lens, and jams there. This is a problem and it can damage your camera.
Do NOT use EF-S lenses on your 6D... ever.
The camera models with APS-C sensors can use ANY Canon EOS lens.... EF or EF-S. But the caemra models with full-frame sensors (such as your 6D) must only use "EF" lenses... never an "EF-S" lens.
Canon specifically designed the camera body so that EF-S lenses will not mount on the camera. You would really have to try to force that lens onto the camera. You should never have to "force" a lens onto the camera. A lens designed to work with your camera will easily fit onto it's lens mount without being forced.
Tim Campbell
5D III, 5D IV, 60Da
