Question: FF DoF vs APS-C

rogerstpierre wrote:

Can anyone explain this to me, as I don't understand: It is widely accepted that APS-C has more DoF than FF because of the sensor size difference, hence leading to the hole argument of FF DoF advantage.

But in the case of a Pentax APS-C camera, where it is using the same FF lens, with the same registration distance, the same focal plane, the same image circle, i.e. all parameters are equal, except that the APS-C sensor is the equivalent of masking the edges around a FF sensor, why would the image projected and recorded on the APS-C sensor by any different than the corresponding area on a FF sensor ?  Makes no sense. I can understand that if you compare an image taken with a 55mm on FF to the same image taken with a 30mm lens on APS-C (i.e. same field of view) that the DoF are different as the FL is different, but both images taken with the same 55mm lens should have the exact same DoF no ?  Am I missing something or is this whole argument about FF DoF advantage just a case of twisted logic ?  i.e. there is no difference in DoF between FF and APS-C.  All that changes, is the distance required between camera and subject to achieve the same result !

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Roger