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Thread: Scheimpflug Rule

  1. #1

    Join Date
    Oct 2006
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    Unhappy Scheimpflug Rule

    After I join 4 x 5 world, I know a little bit from the above rule. But I still have several questons.

    For example, I am going to take a long building, not tall. According to the rule, to obtain overall sharpness along a subject plane, I only need to swing the lens plane or the film plane, so that the subject plane, lens plane and film plane converge at a common point.

    Now the questions are
    1. Does the degree of swing affect the depth of field with same aperture?
    2. Does the depth of field affect by different apertures with same degree of swing?
    3. Ebony homepage said that some of their camera have asymmetrical movement. The asymmerical movement will give better sharpess of subject. Any comment?


    Thank you brothers

  2. #2
    Moderator Ralph Barker's Avatar
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    Re: Scheimpflug Rule

    When using tilts or swings, DOF essentially becomes a wedge, with the near and far points on either side of the plane of sharp focus. Thus, it becomes a little difficult to predict in the usual sense. Often, it is best to carefully check focus on the ground glass as you stop down the lens.

  3. #3

    Join Date
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    Re: Scheimpflug Rule

    1. No. The Scheimpflug rule doesn't relate to depth of field, it relates to placing the plane of focus in a position such that everything lying on the plane will be in sharp focus. Depth of field is affected by only three factors, aperture, focal length of lens, and distance from subject.

    2. Yes. As noted above, aperture affects depth of field.

    3. I don't think so. While the Ebony cameras I owned didn't have asymetrical movements, it's my understanding that having them is a convenience factor and has no effect on sharpness, i.e. someone without asymetrical movements can get to the same place as someone with them, it just might take longer. However, someone who owns a camera with asymetrical movements might provide a better answer.
    Brian Ellis
    Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
    a mile away and you'll have their shoes.

  4. #4

    Join Date
    Nov 2003
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    1,218

    Re: Scheimpflug Rule

    Quote Originally Posted by raylamsk View Post
    After I join 4 x 5 world, I know a little bit from the above rule. But I still have several questons.

    For example, I am going to take a long building, not tall. According to the rule, to obtain overall sharpness along a subject plane, I only need to swing the lens plane or the film plane, so that the subject plane, lens plane and film plane converge at a common point.

    Now the questions are
    1. Does the degree of swing affect the depth of field with same aperture?
    2. Does the depth of field affect by different apertures with same degree of swing?
    3. Ebony homepage said that some of their camera have asymmetrical movement. The asymmerical movement will give better sharpess of subject. Any comment?


    Thank you brothers
    First, one quibble. Any three planes in space, no two of which are parallel, intersect in a point. The significance of the Scheimpflug rule is that the three planes intersect in a LINE. If you take a two dimensional cross-section---as most diagrams do---that line shows up as a point, but if you really want to understand this material, you have to think three dimensionally.


    1. Expanding a bit on what Ralph said, the degree of swing
    DOES affect the DOF.

    You can determine the depth of field, but first you have to understand about the hinge line. That line is where the subject plane intersects the plane through the lens (approximated as a point) parallel to the back. The hinge distance is the perpendicular distance from the lens to the hinge line. The hinge distance is directly related to the tilt angle by a formula, but you can estimate it visually by taking your eye off the gg and looking at the orientation of the subject plane in the vicinity of the lens. The DOF wedge Ralph referred to is bounded by two planes which intersect at the hinge line. The subject plane splits the wedge in "half" in the sense described below.

    The size of the wedge is determined as follows. First find the hyperfocal distance for your focal length and aperture (and your choice of coc). You can use any of the DOF calculators or tables that are relevant to your format to do that. Then, for a swing, at the hyperfocal distance from the lens, the DOF wedge extends laterally on either side approximately the hinge distance. At other distances the extension grows or contracts proportionately as the ratio of the distance to the hyperfocal distance.

    From this you see why the degree of swing affects the size of the DOF. As I said, the hinge distance is determined by the swing angle.. To move the subject plane closer to the lens, you have to increase the swing angle and decrease the hinge distance. This makes the wedge narrower. To move the subject plane further from the lens, you have to decrease the swing angle and increase the hinge distance, and that opens up the wedge wider. (For swing zero, the hinge line is at infinity and the above rule holds but doesn't provide any useful information.)

    You can work backwards from the two bounding planes to determine an aperture that will work. this is a variation of the near-far point method described elsewhere in the lfphoto webpage for no swing or tilt. Having fixed the swing, you focus first on a point you want in focus to the right and then on a point you want in focus to the left, and you measure the distance on the rail between those two positions---it is called the focus spread. The appropriate aperture can then be chosen by one of the methods described in the lfphoto webpage. One method that will work for choosing a minimally acceptable aperture is to multiply the focus spread by 10 and then divide by two. That gives you the f-number. But this is just barely acceptable and because of lens aberrations and other problems may not yield acceptable results. so one should stop down somewhat further. Hansma's method gives an optimaum result balancing defocus and diffraction.

    2. seems to be the same question as 1.

    3. Asymmetrical movement, to be distinguished from axial movement, may make it easier to choose the proper swing angle, but it won't yield sharper results. The so-called near-far method will give you what you want wherever the swing axis is. You choose near and far points which you want in the exact subject plane. You choose a modest tilt, say about 5 degrees. You then focus on the far point and refocus on the near point. If you have to move the standards further apart to do that, you increase the swing. If you have to move them closer together, you decrease the swing. You then repeat this process until you get it right. After some experience you should be able to do it in just a few iterations.

    The advantage of asymmetric movement is that there is a reasonable chance that something of interest will be "on" the swing axis. In that case you can just swing and observe other points coming into or going out of focus while the interesting point stays in focus. This reduces the need for refocusing while adjusting the swing angle. But if you are willing to do that, you should be able to get it right almost as quickly by the method I outlined above.

  5. #5

    Re: Scheimpflug Rule

    There is another more unconventional use for this. When doing selective focus, the wedge effect can create some interesting results. Setting up the camera, visualizing a scene with selective focus, and getting everything the way you hope others will find compelling, can often be tougher than using movements to create sharper overall images.

    Leonard gives a great explanation of these things. You can use that to create an impression of more aspects in a scene being in focus, or you can use that to do the opposite. Sometimes it is fun to experiment and try both approaches.

    I came into doing more selective focus after viewing some large format shots that rendered scenes much as one might see miniature models. Since I was mostly stuck to ground level, and in a busy urban environment, I chose this direction to help isolate details. It can often be a stranger and slower way to work.

    Ciao!

    Gordon Moat
    A G Studio

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