Comment by foldr

The first quote is clearly talking about depth of field, not depth of focus. See also what I quoted in my original comment.

Depth of focus isn’t really relevant to the rendering of an image (except insofar as you want your camera to be built to sufficient tolerances that a sharp image can be obtained when desired).

I assumed you were using “depth of focus” to mean “depth of field”. If you really meant “depth of focus”, then I would say you are mistaken in thinking that the author’s goal is to obtain a narrow depth of focus.