Nancy, thanks for the link. The info given there sheds some light on the subject
Ellis, I agree that much of time this doesn't have a practical effect in general photography, but in some cases it does. As Bob mentioned, a reduction in focal length changes the angle of view at close focus. And at least in the case of my Canon 100mm macro lens, the aperture changes too, increasing the f-stop by 2 stops, which at the small apertures typically used in macro work also increases diffraction. Looking through the rear of the lens, I can actually see the diaphragm blades moving and closing down as the focus is changed. But the main reason I'm researching this is not for a practical purpose, rather for my own edification as to what is happening and why.