If one were to place on a multi-camera bar a 20D and a 5D using the same mm lens at the same f stop the picture would be exactly the same except for the crop factor of the aps sensor, right? At the same time, the DOF is only a function of the distance from the sensor to the subject at a given f stop, right? The shorter the distance from the sensor to the subject the shalower the DOF , the longer the distance the deeper the DOF. Have I got it?
This is an excellent example of the sort of statement that brings together subjective and objective concepts of DoF which are the source of much confusion.
....the picture would be exactly the same except for the crop factor of the aps sensor, right?
That's almost like saying, 'these two pictures are exactly the same apart from the fact they are different'.
If you compare a 5D image with a 20D image in the manner you suggest, what is the impression you get if the 5D image, with its wider FoV, has a few out-of-focus rocks in the foreground which have been cropped out of the 20D image?
Subjectively, whether 2 images are completely different or similar, the image containing blurred objects in the foreground, background or to one side, will be perceived as having shallower DoF. If you crop the 5D image so the FoV is exactly the same as that of the 20D, you are comparing a 5mp image with an 8mp image. You don't have to be a genius to work out which image is likely to be sharper.
To the extent that the 20D image is sharper than the 5D image at the plane of focus, the 20D image will have a shallower DoF.
Your above statement would be more true if the comparison was between a cropped 1Ds2 image and a D60 image, same lens, same f stop, same distance to subject, same pixel density.
A couple of days ago I used my TS-E 24mm with the 5D to get an extended panoramic shot of one of these magnificant ruins in the jungle at Siem Reap (Preah Khan, actually), by extending the width 22mm using shift. Back at the hotel, having downloaded my day's shooting to the laptop, I was dismayed to find the corners and edges were not sharp. I've been used to using this lens with my D60 and 20D where there's no problem with resolution fall-off at the edges.
It's really not acceptable. I wish Canon would upgrade this lens. It's expensive and we deserve better. I felt compelled to return to the same venue this morning and reshoot using my 20D with the same lens but from a greater distance.