The mirror, ever since the 20D was the problem... If i'm not mistaken, the 20D was the first camera that allowed such as lens with a recessed back to fit inside with some tweaking of the mirror placement. It's been a good 8-9 years ago so it's hard to remember vividly the details... the 10D, while being a crop camera, because the mirror is in the same spot as all the other FF cameras, cannot take ef-s lenses. I have heard of work arounds such as removing the back element of the lenses, and some 3rd party lenses dont have an extended back which may or may not work with the sensor, albeit, the image circle is not big enough for FF so you get what you get from that lens. I'm not sure how nikon does it, or if their lenses are designed similar to canon's in the rear of the ef-s lens, but unless canon implements a mirror to either slide back a bit or has a mirror lock up/live view only with ef-s, I dont know how they could work around it without redesigning lenses, which i dont see happening.