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Thread: Scheimpflug Rule when near is higher than far

  1. #11

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    Re: Scheimpflug Rule when near is higher than far

    Quote Originally Posted by Ole Tjugen View Post
    I use front drop for quite a lot of my landscapes, simply because all the interesting bits are below me!
    Isn't this the same as pointing the camera down and tilting the back? I think we're describing the same thing but just expressing it differently. I could have been more clear; when one talks about rotating the plane of focus, I guess an obvious question is, "Relative to what?". I meant relative to the normal position perpendicular to the line of sight, and should have said so. It's certainly possible to have an angle greater than 90 degrees between the back and the PoF, as often happens when tilting the back rather than the lens.

  2. #12
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: Scheimpflug Rule when near is higher than far

    Agreed.

    So even without tilting the back, it is possible to get an angle greater than 90 degrees between the back and the PoF. You may run into coverage problems, but that has nothing to do with this question.

    Some front drop is also quite common in this kind of situation, since all the landscape is "down". That helps a lot on the coverage.

    You end up with something like this (Scheimpflug'ed with front tilt & drop, back vertical):

  3. #13

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    Re: Scheimpflug Rule when near is higher than far

    The only time you would tilt the front standard backwards would be to more closely align the plane of focus with a plane of the subject when the plane of the subject was above you - such as a ceiling.

    steve simmons

  4. #14
    Joanna Carter's Avatar
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    Re: Scheimpflug Rule when near is higher than far

    Well, After 2 1/2 hours, I finally managed to get everythng in focus on this image. I ended up with the tiniest possible bit of back-tilt on the front standard. If you look at the tables that Merklinger gives, you will see that, theoretically it should be possible to get around 85° of acceptable focus; tilting backwards by that minute amount should just about get the foreground in whilst allowing right to the horizon, although the sky might not be as sharp as required. In my shot, the sky was not relevant as I also used front drop to include the manhole cover and lose a very bright hazy sky.

  5. #15

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    Re: Scheimpflug Rule when near is higher than far

    Let me try to mediate between Jeff and Ole and perhaps add some enlightenment to the general problem. This depends on the geometry, which I will try to describe in words, but understanding it will probably require drawing some diagrams which I will leave to you.

    You have to keep in mind that there are two things which set the subject plane. Let me assume for the sake of argument that the back is vertical and we are tilting the front standard. (One can do a similar analysis for other orientations.) The tilt angle sets the position of the hinge line. If you tilt downward, the hinge line will be below the lens, which I believe is what would be desired in this case to keep the foreground in focus. But the actual position of the subject plane is then set by the position of the back relative to the lens, i.e., by focusing, which to simplify the argument, I will assume is done by moving the back. As you move the back closer to the lens, the Scheimpflug line moves up. When it is level with the hinge line, the subject plane is horizontal, but as you move the back closer to the lens, it moves above the hinge line, and the subject plane tilst downward..

    Now consider another important plane, that parallel to the lens plane and at a distance equal to the focal length in back of it. This plane also intersects to film plane in a line parallel to the Scheimpflug line, and no point in the film plane below that line corresponds to any real subject point. So the frame in the film plane will have to lie entirely above that line. That means that as you focus closer in an attempt to increase the angle the subject plane makes with the horizontal, you will have to raise the frame, i.e. raise the back. Thus, how far you can raise the back, or equivalently drop the front (and perhaps compensate by raising the camera), will determine how steep the angle between subject plane and the horizontal can be.

    Now in practice, you would use a near, far point method to determine the tilt and the position at which to focus in order to put the subject plane where you want it. So in effect the tilt angle and the focus position, which sets the Scheimplfug line, would rule. You would then have to raise the back so the part of the subject plane you want in the picture is in the frame. Whether you can do that or not in any given situation will be determined by the details of the scene and the capability of the camera.

    One last comment. I've lost track of the original question was, but if the diagram is any indication, it does appear that one might not be able to get the depression in the middle ground in good focus as well as the immediate foreground and distance rise. That would depend on whether after stopping down sufficiently, the wedge shaped DOF could encompass it.

  6. #16
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: Scheimpflug Rule when near is higher than far

    I'm sorry, Leonard, but that didn't make me any wiser.

    This one did, though: http://www.trenholm.org/hmmerk/#FVC

  7. #17
    Joanna Carter's Avatar
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    Re: Scheimpflug Rule when near is higher than far

    I find this page http://www.trenholm.org/hmmerk/HMArtls.html with the little movies to be very useful in visualising what is going on.

    Oh, and is this image anything like what you are trying to achieve?

  8. #18
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    Re: Scheimpflug Rule when near is higher than far

    While I understand how the scheimpflug principle works, I usually use a little conceptual trick to do it in practice. Instead of caring about the 3D nature of the scene, I look at the first point on my ground glass and focus on it, then see what I have to do to focus on the second point, and then arrange the tilts so that the lensboard - ground-glass distance changes appropriately for the second point. Once I've got the first tilt estimate done, I'll check the two points again and see which way I have to adjust my tilt to get it right... and iterate from there until it's exactly set.

    E.g., with the typical example where you need front tilt forward, on the upside down representation on the ground glass, the bottom of the glass contains the object which is further away, and so if I've focussed on the near object (top of the glass), I have to pull the front standard back (less extension) to focus on the further object (bottom of glass). In my short hand mental notation, this means the bottom of the ground glass has to be closer to the front standard, so front standard tilt foward moves the bottom of the front standard relatively closer to the ground glass; the part of the scene that requires less bellows extension gets it by virtue of the tilt (in other words, tilting the lens forward gives more extension to the part of the scene on the top of the ground glass where the closer objects reside, and so they get in focus). I don't know if this is a strictly correct description of why the scheimpflug principle works, but it's a practical method that works for me.

    A diagram would probably help... but basically, the idea is that if an object on the left of the GG requires that I focus further out, I swing the front standard to that the left side of it is further from the GG.

    If you work it out, you'll see that this shorthand is actually a perfectly valid way of looking at it and is applicable to all situations. It won't fail; it's just a camera-centric way of conceptualizing things instead of a scene-focussed method. It works in the case of a planar region that you want in focus just as well as with two objects. If you wanted a tabletop in focus, you'd just focus on the close side of it, figure out what movement you need for the far side, and iteratively adjust focus & tilt until you hit the magic angle.

  9. #19

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    Re: Scheimpflug Rule when near is higher than far

    Quote Originally Posted by Ole Tjugen View Post
    I'm sorry, Leonard, but that didn't make me any wiser.

    This one did, though: http://www.trenholm.org/hmmerk/#FVC
    I'm familiar with that but it doesn't show the diagram I had in mind. The diagram of the hinge rule show there would illustrate my point if you add a line for the film plane and another line one focal length to the left of the lens plane and parallel to it.

    Unfortunately, it just shows the situation where the subject plane is pointed upward. Somewhere Merklinger has an animation showing how the subject plane hinges on the hinge line as the rear standard is racked back and forth, but I didn't see it at this web site.

    Perhaps if I have the energy, I will post my diagram.

  10. #20

    Re: Scheimpflug Rule when near is higher than far

    Quote Originally Posted by Leonard Evens View Post
    ...Somewhere Merklinger has an animation showing how the subject plane hinges on the hinge line as the rear standard is racked back and forth, but I didn't see it at this web site.....
    Is this the one?
    http://www.trenholm.org/hmmerk/ViewCam.mov

    It's on the page that Joanna linked to.

    Good light! - Hening
    Hening

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