[midlife_crisis]

I got the following from here: http://www.imaging-resource.com/PRODS/E30D/E30DA7.HTM

The Automatic Depth-of-Field mode (A-DEP) uses all nine autofocus zones to determine the depth of field in the active subject area. Once it has determined the range of focusing distances present across the nine zones, it automatically computes the combination of aperture and shutter speed needed to render the nearest and furthest points in sharp focus. This is a remarkably useful feature, even for professional photographers. In many situations, you want to keep several subjects in focus, while at the same time trying for the highest shutter speed (largest aperture) that will permit that. In practice, faced with such situations, I've usually resorted to just picking the smallest aperture feasible and hoping for the best. With the 30D's A-DEP mode, the camera takes the guesswork out of this process and gives you the fastest shutter speed it can manage while still keeping things in focus. (In playing with this, I was often surprised by how large an aperture in fact would work. I frequently would have chosen a much smaller aperture to stay on the safe side.)

It's safe to say I've learnt somthing today.