The above is what happens with a 400mm lens as the photographer foot zooms, switching from one sensor format to another with the requirement to (1) frame the object identically AND (2) maintain a constant DOF.

Please note the 1DmkIII's crop factor is actually 1.26.

Referring to the "Delta Stops" column, when compared to smaller sensor sizes (e.g. 1DmkIII), a large sensor size (e.g. the FF 1DsII) requires that you stop down the lens by approximately 2/3 stops to achieve the same DOF; of course, this means you must throw away light which forces you to shoot at either a slower shutter speed or higher ISO speed setting.

Similarly for a 1.3X (1DmkIII) format versus 1.6X (40D, or in you case 7D) the Delta Stops values (-0.681 and -1.382, respectively) must be subtracted to determine how much more one must stop down using a 1.3X versus 1.6X crop factor body; in this case the answer is about .7 stops.