Upload & Sell: On
Let me suggest some real world ways to deal with this. The fact of the matter is that very, very few people actually calculate this stuff. More likely we work by hunch, by leaving some extra leeway, or by checking the image directly. In addition, essentially everything related to DOF includes a highly subjective component: How large will you print? How sharp is sharp enough for your subject? While the calculations have a veneer of accuracy - they are based on "data!" - it just isn't that simple.
Some real approaches used by many photographers:
- Use a slightly smaller aperture than you think you might need. If yo shoot full frame, there is little or no reason to avoid f/16, for example.
- In general, your idea of focusing "1/3 of the way into the scene" can be close enough in many cases. Basically, focus between your near and far subjects, but a bit closer to the near subjects.
- If you are this concerned about image resolution, I'll bet you are shooting from the tripod, right? If so, use live view to focus manually. With "exposure simulation" activated, use 10x magnification, press the DOF preview button, and scan around the scene, carefully inspecting your near and far subjects directly.
- Use focus bracketing if you need exceptionally deep "depth of field." This involves making a series of exposures, using a tripod, in which the focus point is shifted in each exposure. The exposures are the combined in post by one of several means I won't describe right here.