Let me start by saying that I stand with practise, practise, and more practise, not calculate, calculate, calculate—and not only because no one ever accused me of being able to count above 9. My use of some specifics in examples below reflects my concern to improve my grasp of previously familiar principles in service of better command of the instrument, but not in place of the fun of hands-on experimentation and the work of practice. That said, I have recently reread numerous view-camera books and articles, including, naturally, the Adams series, as well as Sexton, Barnbaum, Bond, lots of posts here and on APUG, etc. My copy of Merklinger’s Focusing the View Camera took its turn, and that leads me to my question.
Whenever circumstances permit me to return to 4x5 work (necessarily laid aside for a decade-plus now), my primary subject will be portraits with my only lens, a 210. I can foresee some applications of the use of small tilts and swings in the kind of images I have in mind to create, and therefore have been striving to get a good handle on the hinge rule, the angle of the plane of focus relative to distance J, and DOF. My ineptitude in geometric thinking has already led me down a number of false paths in pursuit of grasping what Mr. M. lays out, though I have made progress as well. My interest here, is to ask whether there is a principle or method I am missing for pre-estimating DOF when the lens is tilted, one free of trigonometric functions and multiple-step calculation.
In simple, I think I understand from Mr. M that,
a) for a given focal length, DOF is constant for a given plane-of-focus distance and aperture, whether the lens is titled or not;
b) that when the lens is tilted, however, the DOF is measured along and perpendicular to the plane of sharp focus (PSF), rather than along the lens axis. Thus, it will always be (technically) some amount less than lens-axis DOF (although its angled wedge field may sharply encompass far greater distance ranges within the bounds of the image than non-tilting provides, which is one reason we use it, of course). First of all, have I got that much correct?
Here’s an example of a portrait subject seated at a table, facing the camera, eyes 6 feet from the vertical focal plane. Let’s say his folded hands on the table extend toward the camera about a foot in front of the eyes. With my 210 at f/16, total DOF will be 8.5”. A modest 2-degree forward lens tilt, for instance, extends J to 20 feet, which orients the PSF, from hinge to eyes, at an angle that (by my rough pencil-and-ruler drawing) would tend to intersect the subject’s hands about 2 inches from the front. The ~4” of near DOF would therefore just include the hands, since, measured perpendicular to the PSF, it extends horizontally only about 3 inches; the far would fade toward the elbows. However, since the front of the hands actually lie only 5'2" await, not 6' , total DOF at the front will be only about 6", hence perhaps 2.5" on the near side at that line.
f/22 or something in-between might be an option, of course; again, I’m just trying to make sure my thinking is straight, not fixate on numbers. At present, with very limited actual practice time available, it helps to work through such mental exercises, specific to my equipment, so that I go into my practice time prepared. With one lens, memorizing approximate total DOF at several approximate distances, and knowing how total DOF varies with distance and f/ stop, so that a single distance and f/stop can be offer quick estimates of what changes could be made, is useful.
One addition: Mr. M. says that the PSF pivots on the hinge when the back, not front, standard is moved, though he also says that refocusing with the front doesn’t materially affect the matter. I experimented tonight with three homemade high-contrast 18" vertical rulers I lined up on my dining table, at 5.5-, 7.5-, and 9.5-foot distances. I found that the angular changes of the PSF by moving either standard to focus on several selected marks, were virtually identical. Is my set up simply too small to make a difference?
My thanks for contributors this forum, which continues to be an invaluable reference and a sometimes inspirational source of imagery.
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