I’d like to pose a question I asked once before, stating it in a more specific way now, to see if someone can add clarity to my slightly fuzzy view. It pertains to quantification of depth-of-field. While in practice we use the image on the ground glass, there are times when it's hard to see, e.g., indoors with small apertures. Let me emphasize at the outset, that this is just a matter of curiosity for me regarding the principle involved. My work does not (thank goodness) require the sort of measurement that could be undertaken to determine the information suggested below. I'm just interested in understanding the general forces at work here a bit better, if this be possible without resort to cosines, integrals, etc.
In elaborate optical calculations I can not begin to follow, Harold Merklinger in his exhaustive treatments makes this point, among others: When tilting or swinging the lens and/or back, the DOF is 1) always equal on either side of the plane of focus 2) the DOF extends into the distance along straight lines, forming a constant angle.
While I recently discovered that, with camera standards parallel, the DOF stays nearly equal in front and behind the plane of focus to a much greater focused distance than I had previously thought (e.g., to nearly 10 feet with a 210mm lens stopped down as far as f/16), the rate of increase of DOF per unit of subject (plane of focus) distance accelerates rapidly at a certain point, forming a non-constant-angle, "trumpet bell" sort of curvature along its outer limits, and the increase of DOF beyond the subject compared with that between the lens and subject increases likewise.
It would seem at first to make sense that, with a swing or tilt, the total DOF at near and far points of the plane of focus in our image would be the same obtained if the lens, without a movement applied, were focused on the distance given. For example, here's an approximated total DOF chart for my 210 at f/8:
5 2"
6 4"
7 6"
8 8"
10 1'
12 1.5'
15 2.5'
20 4.7'
25 7.5'
My presumption, above, would seem to indicate that if I aimed my lens at an angle to a long wall, such that the near point was 5 ft. and the far one 25 ft. from the lens, and swung the lens to make the plane of focus coincide with the wall, I would get the respective total DOF given above at those two points, which is to say, half to somewhat less than half of it extended on this side of the wall (or strictly half by Merklinger's acocunt). Thus, almost none at the close point, but 3'2" (according to an online calculator, or 3'9" by Merklinger) on this side of the wall at the far point, so that, say, a bushy ornamental plant next to the wall at the far point could fall within the DOF.
However, all that may be dreamland, because, if I'm not mistaken, once we swing or tilt, the DOF decreases as the amount of swing/tilt increases. So, in the example, the DOF would depend on the camera's angle to the wall—and the plot thins.
Again, I'm not interested in wasting anyone's time as if this were critical to me; this is simply a matter of interest to me, and I will appreciate understanding it better. I had noted in my earlier query on this that Stroebel illustrated the principle with curved, dotted lines in The View Camera, but makes no mention of details.
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