A:  The sensor on the Canon 5D is about 40% larger than the Canon 30D which accounts for the increased resolution from 8.2 megapixels to 12.7.  Resolution is not the only story though, the 5D also has larger pixels which collect more light so the camera does not have to amplify the signal as much. This makes for a image with less noise and more dynamic range (the ability to record more detail in the bright highlights and the dark shadows)