I often try to explain the effect that sensor size has on depth of field. This demonstration of three levels of sensor is a great example. Please check it out. Also, note that one is not always better than the others. There are times when you want the depth of field possible with the compact sensor; and that APS-C sensor on the 40D is a real sweet-spot in my opinion. This is a visual argument for the fact that APS-C is with us for good, I think. Of course, the 5D is lovely as well.