I have posted in the past that while I was studying optics at Kodak, that I raised the question about why when I kept the image size and exposures settings the same, I could not get a better depth of field by changing lenses. The optical engineer started with the depth of field equation and made one substitution. Four lines later the focal length was in every term and then was divided out. The result is exactly as Q.G. stated.
SteveWhen magnification and f-stop are constant, DOF will be too, no matter how large or small the frame.