I've been photographing some small but round objects, and running into DOF limits.
My question is, which is better, 35mm or 4x5 for macro? In my opinion, 4x5 is better than 35mm for most things, but as you go more and more final print magnification, 35mm becomes better. At what point is this?
Ignoring film grain and reciprocity failure issues, I understand that using the thin-lens equations for DOF, camera format does not matter when it comes to depth-of-field in the final print. If you want to make a 1-foot-wide print of an orange, then it should not matter what camera format or focal length lens you use to make it. Thin-lens DOF says that it doesn't matter, you can simply stop down the larger format more and always achieve the same DOF as the smaller format.
However, when you consider diffraction, larger formats are hit harder as print size increases (as total object-to-print magnification increases). As magnification from real-world object to printed image increases, eventually larger formats will lose out to smaller ones because you have to stop them down so far to achieve print DOF equal to the smaller formats that diffraction will take over at some point. What point is that? This ignores that there may be more diffraction when you enlarge the smaller format to the final print size.
Is there a rule of thumb as simple as "for FINAL, PRINT magnification greater than 10x, you are better off using 35mm than 4x5"?