The way I learned it, and this is very simple, is that the dof is pretty much dependent on the actual focal length of the lens rather than the format size. In other words, on a 28mm lens, you have generally pretty deep depth of field. If you are using a small format camera and that represents a normal or tele lens, you still have a deep depth of field. A 90mm lens will have generally a pretty shallow depth of field, if you are using a 4X5, that is a wide angle and you have a shallow depth of field, if you are using a 35mm, it is a portrait length and you have a fairly shallow depth of field. I am sure that it is more complicated than that, but it works for me to understand the general issue.