To all, a very useful discussion; one of the things that makes this forum my first stop when I have a question.
In reply to Doremus: I am familiar with the interaction of the interdependence of aperture, distance, and focal length in determining DOF, and thus with the increase of DOF with distance. Having worked almost solely with 35mm for more than a decade before getting beyond introductory school work in 4x5, the DOF marks on the prime lenses I used, the 1/3 to 2/3 rule of thumb, planning location and extent of the DOF by aligning focus-ring min-max distances with the marks, and so on, were something I used and taught. That's why I was surprised, checking before my OP above, that the relative symmetry of fore and aft DOF, which I had thought applied only in close-up work, continued to the distance I indicated. This is also a function of the longer equivalent focal lengths of 4x5 as compared with 35, which hadn’t occurred to me in this respect.
To restate for anyone else reading this, my interest is grasping functional principle, not plunging into the optical mathematics relevant to my query (or quarry). That said, calculating with the Points of Focus online DOF calculator set to d/1500, both with and without a specified COC, the linear progression of increase of total DOF you cite above, does appears to hold (up to a certain point), and indeed, to a much greater extent than I had thought: Revisiting the calculations, with plane-of-focus distances of 5, 10, 20, 40, and 80 feet (with a fixed aperture and focal length), each doubling of distance increased the total DOF by a factor of a bit more than 4. Live and learn. When I doubled the 80 feet to 160, however, the increase was 12 times greater. This distance is close to where the calculator resorts to Infinity (above 180 feet); I just mention it for interest's sake. (Edit: I should add for emphasis, that I was calculating for a 210mm lens; with a 135, the factor acceleration becomes obvious between 24 and 48 feet, because the "infinity" point for DOF calculations is reached sooner.)
Your third paragraph is the one that answers my question: my assumption, as elaborated, that DOF for a given distance (for a given focal length and aperture) should remain constant with movements, is true, and my supposition, that the degree of tilt/swing reduces DOF for a given distance, is false. That's what I wanted to know. Thank you. It's odd to me that the first, to my knowledge, is not simply stated anywhere in the literature and teaching I have encountered over decades, including in any of the Adams books, Stroebel, or maybe two dozen other books and resources on LF movements I have read. It makes me wonder if I am the only one who has puzzled over it and sought the answer.
I have been using the near-far focus-spread technique since I first saw your mention of it in a reply on the forum about a year ago. I copied it, rounded off values, and printed out a copy of the chart which I keep among several laminated, quick-reference items in my bag.
Regarding others' thoughts on favoring sharp focus on the far rather than near, interesting food for thought. If I am not mistaken, Adams opined oppositely, feeling that a slight softness of the far were less disturbing than the opposite. However, I note that you said "may sometimes" and that your consideration is the appearance of harmony in rendering the image as a whole. Isn’t art wonderful?
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