Unfortunately we CANNOT use all our GREAT Canon lenses with a new digital body. Nikon has interchangeable lenses as do many others. For some utterly stupid reason, Canon doesn't offer that. Pisses us off immensely!
Fortunately, I'm here to tell you why! When I worked at the pro-photo supply store in Dallas, I was told the following. When Nikon decided to go with an autofocus line of cameras, they wanted to make their bodies backwards compatible with their older lenses. This meant that they were forced to make their new shiny round square so the cameras would be able to fit the old square lenses (so to speak). The result was that the motor to turn the auto focus parts was on the camera body and connected to the lens via a little screw head that turned. The result (in this guy's opinion of the high dollar Nikons I tried) was a louder, slower auto-focus then Canon. Canon, on the other hand, said "screw it" we are going to create a new line of autofocus cameras from the ground up (beginning with the EOS 650 in 1987). Their autofocus drive was on the lens body itself and the result (once again in my opinion) was a faster quiet autofocus. That was in 1998-99 when I was working there, so the newer Nikons (to my understanding) don't use that screw mechanism and are more quiet and faster than they were. They may even being as quiet and fast as Canon now, but I have thrown in my lot with Canon years ago. So that is why you have to leave your old lenses behind. It wasn't a digital-versus-analog thing; it was the advent of Canon autofocus. We had a Canon Rebel that shot film (still do) and at some point I think it was dropped because it doesn't "lock on" when auto focusing anymore unless it is pointed at an extremely bright light. Manual focus is fine, but it doesn’t have a split ring focusing apparatus in the middle making it difficult. We bought a Canon Rebel T3i a few years back and it even got thrown (long story) and it works like a champ.
Not so much a detractor still of Nikon, but I have a bunch of Canon gear and I like it. Were I to start from scratch today I would have to think about it and borrow other people cameras to make a sound decision.
And now you know. . .the rest of. . .the story!