In basic photo, we [unfortunately] learn that D of F is directly affected by three things: 1) aperture, 2) distance from subject, and 3) focal length of lens. Additionally, we also learn sometimes that film format affects it. Actually, it is only 1) f stop, and 2) magnification. Distance from subject and focal length of the lens can affect magnification, and thus affect D of F. Given the same angle of view/composition, film format also affects it indirectly by affecting magnification. However, they do not do it directly, as we learn in the basic photo classes and books. Changing either distance or focal length or film format will have no effect on D of F *unless* doing so also changes the magnification. The higher the magnification, the less D of F there is.
The idea that D of F is defined as the front-to-back area that "is acceptably sharp" in the print has never set well with me as a useful definition. This means that in a large enough blowup, there could be absolutely no D of F, since even the plane of critical focus can fail to be "sharp", due to "overenlargement". I personally define it as the area in the image that is apparently *just as* sharp as the plane of critical focus. Thus, I view it as a comparison of the plane of critical focus to the rest of the image, rather than a simple definition of what is sharp on the print. Viewing it this way, something can be unsharp, yet still fall within the D of F if it is apparently just as unsharp as the plane of critical focus. Thus, it again comes down to magnification, not just print size. Viewing distance and size of print simply affect magnification (only this time it is your eyes/brain, not a lens/film), which affects D of F.