One way to illustrate what PaulMD saying is to think of a real-world example -the different format cameras Pentax made. Pentax made an adapter to put 6X7 lenses onto the 645. They made an adapter to put 6X7 lenses onto K-mount. A 200mm f4 lens for 6X7, when adapted to 645 or to K-mount cameras, will give the same magnification and Dof with each as the 200mm f4 lenses Pentax made for those mounts.