Pictures are not flattened by the focal length, perspective does that, and that is ONLY the function of distance...
If we keep shooting distances the same, the picture angle the same and use different size sensors (or film), we must use longer focal lengths with bigger sensors. With longer focal length we get less DOF. Simple.
Extreme shallow DOF effets can not be had with digital, try 8x10 film... or IMAX.
35mm format has something for it, thought; the fastest lenses, and that overcomes some theoretical limitations. If we had f1.4 lenses for 8x10", boy...
