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Thread: Regarding depth-of field curvature with tilt

  1. #11

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    Re: Regarding depth-of field curvature with tilt

    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.

    Click image for larger version. 

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    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?
    Last edited by Ulophot; 30-Oct-2021 at 09:56. Reason: additional qualification
    Philip Ulanowsky

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    www.imagesinsilver.art
    https://www.flickr.com/photos/156933346@N07/

  2. #12

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    Mar 2005
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    Re: Regarding depth-of field curvature with tilt

    Often while photographing broad landscapes with multiple possible image planes (like tall hoodoos or trees in an otherwise "flat" landscape), where I know I will need to make some compromises if also trying to attain as much DOF as possible...I will tend to sacrifice a bit of focus at the distant end - especially when those distances might be partially compromised by natural atmospheric anomalies (haze, heat waves, etc.) - where it might be nearly impossible to portray those great distances as "critically sharp" even if focus is adjusted for those distances.

  3. #13

    Join Date
    Jan 2019
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    316

    Re: Regarding depth-of field curvature with tilt

    The size of the blur circle and thus the depth of field is basically just due to similar triangles. Consider a point of light in the object space that is at the far limit of the depth of field. A cone of rays from this point enters the front of the lens. This cone, when it passes through the object plane that is in exact focus, is a circle of some modest size, not a tiny point. So the image on the film is going to be an image of that circle of modest size. This is true whether you have tilts or not. Tilts just change the tilt of the plane of exact focus.

    That probably sounds like a bunch of gibberish in words, so I drew a picture to illustrate it, attached here (click to expand). The sharp-focus rays are in black and the slightly blurred image from the far DOF is in red.
    Click image for larger version. 

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    I didn't try to draw the fact that the near and far DoF in the object space aren't equal, because the triangles have different heights. If your hyperfocal distance is 20 feet (for example) then the near DoF is at 10 feet but the far DoF is at infinity.

    Now, just maximizing DoF ain't everything. As others have said, there may be pictorial reasons to bias the point of focus near or far. Faraway details are small, so one might want to increase sharpness in the back. On the other hand, the foreground of a subject usually draws attention, so one might want the sharpest focus in the foreground and let the horizon go a little softer.

    Because our vision is a simple lens with a huge image-processing brain behind it, no 2-d image directly corresponds to how we see a subject. Usually we don't perceive out-of-focus blur with the eye, but we also selectively direct our attention to part of a scene at a time, which selective DoF sort of mimics.

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