BTW, if you want to get into depth of field scales, there are some informative Internet (and print) articles, and forum discussions. In short, they are often, if not always, "incorrect" for "critical work". My favorite article is the Ken Rockwell one, in which he briefly covers D of F and diffraction, and then explains how to get the sharpest shot possible for a given situation, considering both issues. It is especially helpful for medium and large formats.