Originally posted by Clinton The DoF is the same it has always been. The AoV is changed.
As far as the discussion is going, I kinda I agree with both of you, but in response to this I want to point out that:
With Any Given Lens(tm) when you put it on a larger sensor, you are changing the angle of view.
When you change the angle of view, you change the working distance for a particular type of shot. When you change the working distance, you change the DoF for the same focal length and aperture.
For instance, a head and shoulders portrait. On an APS-C body, an 85mm produces a head and shoulders portrait at roughly 8 feet. At this distance, the lens is producing total approximate DoF of 44mm.
If you shoot the same subject at the same distance with a FF body, you no longer get your Head and Shoulders portrait. To get the same composition as the 85mm APS-C shot, you need to move closer.
Now your working distance is 5.2 feet, and your DoF is 28mm. Much less than it was on the APS-C body, with the same lens, at the same aperture.
So yes, the DoF at a particular distance has not changed, but the distances to get those same compositions is closer on FF with the increased angle of view, and now you have thinner DoF as a result.
And you may slightly disagree with my numbers for the working distances, but this principle is true on all lenses, at all apertures. Moving from a smaller format to a larger one decreases working distances for the same shots, and your DoF gets thinner as a result.