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Thread: A question on hypefocal distances.

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  1. #1
    Landscape Addict
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    A question on hypefocal distances.

    Hi everyone.

    I'm pretty sure I know the answer is yes, but just to be certain..

    In a hypothetical situation, lets say a calculator tells me the HFD for my camera/lens/aperture is 1.3m. That 1.3m measurement needs to be taken from the subject to the film plane?

    I have had conflicting answers from here and there, Some telling me its from the subject to the front element, others from the front element to the center of the lens etc..

    I was certain it was to the film plane.

    Thanks.
    Alex.
    Chamonix 045N-2 - 90/8 - 210/5.6 - Acros100 - Rodinal.
    Alexartphotography

  2. #2
    Land-Scapegrace Heroique's Avatar
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    Re: A question on hypefocal distances.

    In everyday, practical landscaper’s terms, your “1.3 meters” is the distance from the HF point to the camera.

    This makes a lot of assumptions, of course. For example, leveling & movements.

    To be more exact and comprehensive, this answer will get very, very long, heated, and controversial!

    Focusing at this point keeps infinity in acceptable focus, plus everything else that is no closer than 1/2 this distance (.65 meters).

    ---
    BTW, I’ve always noted that AA’s The Camera illustrates this distance as being from the HF point to the front of the lens. This, too, is for simplicity’s sake.

  3. #3
    Lachlan 717
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    Re: A question on hypefocal distances.

    The theory says from the front glass; however, this assumes a single element of no thickness.

    Reality is somewhere around the nodal point, but even this can be hard to define.

    If your closest object for focus is not super critical to the (roughly) 65cm point, just assume the front of the glass, thus keeping infinity in focus.
    Lachlan.

    You miss 100% of the shots you never take. -- Wayne Gretzky

  4. #4

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    Re: A question on hypefocal distances.

    Just to add a little to what Lachlan said: A DoF calculator (and the usual formulae) gives the distance from the lens (specifically the front or first nodal point as Lachlan says) to the subject. If you are using a lens that has a focusing mount it is likely that the distances on the mount will refer to the film plane - subject distance, which is unambiguous because the film plane is usually clearly marked (most movie cameras even have a tape attachment point there to help with measurements). Maybe this is the reason for the different stories you are hearing.

    Best,
    Helen

  5. #5
    8x10, 4x5, ..., Tessina Leigh's Avatar
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    Re: A question on hypefocal distances.

    Quote Originally Posted by Helen Bach View Post
    Just to add a little to what Lachlan said: A DoF calculator (and the usual formulae) gives the distance from the lens (specifically the front or first nodal point as Lachlan says) to the subject.
    Sorry... wrong.

    The DoF calculator gives the distance from the FILM PLANE to the subject.

    It has no knowledge of what lens you're using, nor where the front nodal point (first principle plane) might be.
    In many cases the front nodal point is not even inside the physical lens. It cam be moved by hundreds of mm.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  6. #6

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    Re: A question on hypefocal distances.

    Quote Originally Posted by Leigh View Post
    Sorry... wrong.

    The DoF calculator gives the distance from the FILM PLANE to the subject.

    It has no knowledge of what lens you're using, nor where the front nodal point (first principle plane) might be.
    In many cases the front nodal point is not even inside the physical lens. It cam be moved by hundreds of mm.

    - Leigh
    Leigh, I'm fairly familiar with both the rigorous and the simplified versions of lens formulae, and with a number of analogue and digital DoF calculators, and I believe that you are wrong about this. Emmanuel has already given an excellent answer. I'll give a shorter one. The person making/writing the calculator does not know the internodal space, apart from anything else. If you look at the popular online calculators (at least the ones whose authors appear to understand the subject), they state that they are giving first nodal point - subject distances. You need fewer assumptions.

    If the "front nodal point is not even inside the physical lens. It cam be moved by hundreds of mm" then whoever is designing or writing the DoF calculator doesn't stand a chance of getting the subject-film plane distance accurate: the internodal space will be quite large.

    Here is what the popular dofmaster website has to say:

    "Is the 'subject distance' measured from the front of the lens or from the film plane?

    This question is relevant only for close-up and macro photography. For other photography, any error caused by measuring subject distance from the wrong location is neglible.

    The depth of field equations are derived from the "thin lens" equation, which assumes a single lens element with no thickness. And, subject distance is measured from the thin lens. So, technically, subject distance is measured from the front of a lens.

    However, that doesn't apply directly to a photographic lens. These lenses have many elements, and the front of a lens isn't necessarily the location that subject distance is measured from. The actual location is something called the "front nodal point" of the lens. The location of the front nodal point isn't documented by lens manufacturers, nor is it easy to find experimentally.

    I measure subject distance from the front of the lens. I believe that the nodal point would actually be somewhere between the front and rear elements. But, by assuming it is at the front of the lens, I get a conservative estimate of the depth of field from the calculator. In other words, the depth of field calculation shows less depth of field than will actually be seen in the photo.
    "

    Of course we are lucky: many of the modern lenses we use do have the nodal points given by the manufacturers, not that it matters a whole lot most of the time. It's also quite easy to find if high accuracy is not required.

    I only mentioned it as a possible explanation of the conflicting advice that Alex had heard. There are many reasons why it is an approximate science in addition to those given by ic-racer. (basis for max. acceptable CoC; simplifications in the formulae etc.)

    Best,
    Helen

    PS It's a principal plane, not a principle plane, but I'm sure that you know that.

  7. #7
    ic-racer's Avatar
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    Re: A question on hypefocal distances.

    The hyperfocal distance to the subject for most critical large format systems (small CoC) is far enough away that it does not matter where you measure it. In large format systems I consider the hyperfocal point merely an intellectual curiosity. Best bet is usually to focus on the near object, focus on the far object, split the difference on the standard and set the aperture based on your spread in milimmeters.

  8. #8

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    Re: A question on hypefocal distances.

    The DoF calculator gives the distance from the FILM PLANE to the subject.
    It has no knowledge of what lens you're using


    If this is so, then the guy who designed the calculator is simply illiterate in optics and we strongly recommend him to come here more often and learn

    Various (compound) lenses exhibit various distances between their nodal (or principal points).
    A "caricature" for this can be found in telephoto lenses.
    And all classical models for geometrical DOF refer to the cardinal elements of the lens : foci and principal (or nodal) points. And the hyperfocal distance is measured in object space with respect either to the front focal or the front nodal point of the lens (see below).
    Moreover, formulae for asymmetric lenses differ from the classical model, valid only for symmetrical lenses. So, not only for a general coupound asymmetrical lens you should know the inter-nodal distance, but even more, you should also know the pupillar magnification ratio and you should apply the most general formulae.

    There are different issues there
    1/ the engraved distances on helical focusing rings for small and medium format cameras, by tradition, refer to the object-to-film-plane distance, hence we can trust the engineers to have taken into account the inter-nodal distance for a specific lens. Helicals designed for view camera lenses do not care for the inter-nodal distance which is usually small for wide-angle and standard view camera lenses. And of course regarding DOF engravings, do not care for the pupillar magnification, view camera lenses that you mount on an helical are most often wide angle or standard lenses of quasi-symmetrical design.

    2/ the hyperfocal distance "H" for symmetrical lenses (let's forget about telephotos and retrofocus lenses) can be defined with respect to the front focal point F or the front nodal point.N. The difference between the two lengths is simply equal to one focal length "f"
    In classical models, the "true" hyperfocal distance between the object and the font nodal point N is equal to (f^2/(N.c)) + f where "f" is the focal length, "N" the f-number, and "c" the chosen diameter for the circle of confusion.
    Usually, the term f^2/(N.c) is referred-to as the hyperfocal distance.
    Stricly speaking, we should say : f^2/(N.c) is the "focal hyperfocal distance" and f + f^2/(N.c) is the "nodal hyperfocal distance".
    Of course nobody cares for that. Let's simply say : H = hyperfocal distance = f^2/(N.c)

    3/ Now if you insist on having the true value for the distance between the object and the film plane at the hyperfocal setup, then the total distance is
    H + f + NN' + f + 1/H = (H + 1/H) + 2f + NN'.
    Fortunately enough, "NN'" is most often small with respect to "f", so we can neglect "NN'", and we get the simplified formula actually valid for a single thin lens element : (H + 1/H) + 2f;

    Again, 1/H is most often very small with respect to H, and eventually we get:
    H = f^2/(N.c) is the hyperfocal distance measured to the front focal point F
    H + f is the hyperfocal distance measured to the front nodal point N
    H + 2f is the hyperfocal distance measured to the image plane, when H is much bigger than 1/H i.e. "all the time" for landscape shots.

    As a conclusion, there is a very high probability that those simple formulae are actually in use in most DOF calculators, defining the hyperfocal distance as usual. The software designer should simply tell the user which reference point is:considered : either the front focal point (probably not), the front nodal point (high probability) or the film plane (like for engravings on helicals according to a secular tradition), and which approximations are in use.

  9. #9

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    Re: A question on hypefocal distances.

    Why hasn't anyone mentioned that HFD is a highly variable value and using it to set the focus will only give accurate results only for only one or maybe two reproduction instances?

  10. #10

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    Re: A question on hypefocal distances.

    Quote Originally Posted by genotypewriter View Post
    Why hasn't anyone mentioned that HFD is a highly variable value and using it to set the focus will only give accurate results only for only one or maybe two reproduction instances?
    Eh? Wot? Please explain further, and while you're at it show us where Prof. Dr. Bigler and Ms. Bach went wrong.

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