Originally Posted by David C. Williams
If you move back and zoom in to the same framing of the objective, you dof does not change. What it does is it narrows the field of view behind the objective, expanding a section of the original background view to fill the entire background in the zoomed view. The dof has not changed, but the lack of focus detail which was always present in the original view is magnified and becomes more apparent.
Opening the iris will decrease dof. Zooming will decrease dof, but if you also move the camera back, and zoom to the same objective framing, you end up with the same dof.
This is sort of true. nothing in the image will be more blurred by moving back and zooming in, the background blurry bits will just be larger. It's also strictly speaking incorrect because any element behind the point of focus, where the amount of blur was just below the resolving ability of the viewer will be enlarged (assuming the subject at the point of focus is sized identically in the frame) and then the blur will be just perceivable and as such the depth of field will be reduced. However I think we are both being a bit pedantic (I know I am). I suspect in this case what is being asked about is the "look" or "appearance" of shallower depth of field and that can be achieved by moving back and zooming to change the perspective of the scene.