Where to compare dof between the 6D and 1D4?

The sensor doesn't give you greater DOF, it gives you a wider field of view. So it doesn't make a 24mm lens have less DOF, it makes it look like a 24mm lens instead of the 30mm lens it looks like on an APS-H sensor.  This also means that if you have a 35mm lens on a 6D, to get the same field of view on a the 1D Mk 4 you'll have to use a 28mm lens or stand farther back, either one will reduce the DOF (and the latter will also change the perspective).

DOF is effected by 3 things in the camera: Focal length of the lens, distance to the subject, and f/stop. Sensor size does not come into play except that changing the sensor size will likely mean you will change the focal length or the distance to the subject. Depending on what changes you make the DOF might be affected differently.

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\~K