:I sympathize with the problem, but this is the conventional way it's done.  The "outside the box" or "von Rohr's method" approach is seldom seen in literature on DOF, so we shouldn't introduce it as the main approach here.  Also, let's don't introduce the unneeded neologism "bokeh" in the lead; you can refer to later if you want to, but it's not part of any traditional discussion of DOF.