I think that what you are looking for is the reach when you say 12x zoom. Look on the camera, on the lens, it usually says some measurement, equivalent to 35mm. I'm pretty sure that this means that if you were getting a lens for a full-frame, you would need a lens that has that max amount of millimetres. So, let's say your camera has 35-420, you need 420mm on the long end on the lens to get the same reach as your S3. But, since the 400D has 1.6 crop factor, I think you divide your max amount by 1.6 to get the amount that you need. I saw this explanation in a thread a while ago, don't remember which one, so I explained. Have fun with your camera!