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  1. #1
    Sirius Glass's Avatar
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    Lens Convergence?

    These are two photographs that I took learning to correct for convergence. The negatives were quickly flat scanned saved; then GIMP was used to reverse the images and adjust the contrast so that they would be viewable.

    Both negatives were taken with a Schneider-Kreuznech Angulon f6.8 90mm lens. In both photographs I thought that I had corrected for convergence, but on the right of both of them there is convergence. The one of Haper's Ferry has slight convergence and the one of the church has a larger convergence.

    Are the convergences an artifact of the lens or an Operator Assisted Failure [OAF]?

    Steve
    Attached Thumbnails Attached Thumbnails Harpers Ferry.jpg   Church.jpg  
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

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    I leave the digital work for the urologists and proctologists.

  2. #2
    BradS's Avatar
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    Neither really looks like lens distortion. Both really just look like you did not have the camera back perfectly vertical.

    If you are sure that you did, there may be a mechanical issue with the camera or film holder itself - but these seem very unlikely.

  3. #3

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    OAS is the likely culprit.

    If you leave the lens centered over the film with all controls at neutral and keep the lens axis horizontal (you can check with a sprit level), then the verticals should come out vertical on the image. This isn’t perspective correction. Rather it’s prevention of distortion in the first place. But the composition might not meet your requirements. One possibility is to shoot horizontally so that no distortion occurs, and then crop out what doesn’t fit the image you want.

    Often we have no choice but to aim upward. That necessarily makes the verticals converge at the top.
    The difficulty in seeing things near the edge of the frame, especially with a WA lens, makes correcting architectural perspective tough. About the best you can do is learn to stay under the completely dark focusing cloth long enough until your “night vision” sensitivity increases enough to see the darker parts of the screen for accurate correction.

    To make this work you must have the GG and your head well tented so that NO light reflects upward from the ground. That’s easy to state, but not so easy to accomplish.

  4. #4
    Diapositivo's Avatar
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    The first image looks tilted to the right to my eyes, and the second tilted to the left. This possibly influenced the subsequent perspective "correction" work. Maybe the ground was soft and after levelling the camera there was a small movement which you did not notice.

    You can clearly see that the camera is not levelled because an imaginary central vertical line, which is vertical in nature, and which is in the exact centre of your picture, must be vertical (all other lines will converge to a point "above the frame" if the camera is looking "upward", or below the frame if the camera is looking downward, but a central vertical line is always vertical, and if it isn't that means the camera is not properly levelled.

    This is very hard to judge by eye as for the eye verticals are always vertical. I mean, if you brutally tilt a camera 20° to the right an look through its viewfinder, the horizon will be horizontal just the same but the resulting frame will obviously be tilted 20° to the left.

    Small differences are very hard to judge. Bubble levels are decently precise, but not overly precise. One can easily make a mistake of 0.1 or even 0.2 degrees using bubble levels. I don't understand why don't they make them more precise. It probably has to do with tolerances in installing the bubble on the tripod head (or tolerances in perfect geometry of the cameras).
    Fabrizio Ruggeri fine art photography site: http://fabrizio-ruggeri.artistwebsites.com
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  5. #5
    Diapositivo's Avatar
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    Fabrizio Ruggeri fine art photography site: http://fabrizio-ruggeri.artistwebsites.com
    Stock images at Imagebroker: http://www.imagebroker.com/#/search/ib_fbr

  6. #6

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    Yaw free controls are best. With some view camera setups as you correct one part another part goes out, so you have to adjust by successive approximation until you get everything lined up the way you want to. Some good advice above.

  7. #7
    David A. Goldfarb's Avatar
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    These shots don't need very complex movements, and the 90/6.8 Angulon doesn't produce this kind of distortion, so my guess is that you were probably trying to overcorrect, and that caused more problems than it solved. For shots like this, you should only need to aim the camera, level the camera on the tripod, and use some front rise/rear fall to frame, maybe shift if there are issues about where you are able to put the camera, or you want to look down an open corridor or something like that, and you shouldn't need any tilt or swing (except with both standards parallel if you need indirect rise/fall/shift, which you shouldn't with the modest image circle of a 90/6.8 Angulon on 4x5"). While a yaw-free camera is nice, these kinds of shots shouldn't put you in a situation that produces yaw (i.e., tilt and swing on the same standard).

    Also, be careful with the 90/6.8 Angulon, because the circle of good definition is considerably smaller than the circle of illumination. See how you're going blurry in trees and the bell tower in the first shot? That's either more rise than the Angulon is good for, or unnecessary tilt or both.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
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  8. #8

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    steve

    does your ground glass have a grid on it ?
    you don't need something expensive, or to draw lines in pencil ...
    i printed out a grid in mac draw years ago, and printed it on paper
    and then went to my neighborhood copy shop and printed it on a piece of
    clear plastic. it cost about 10¢ ...
    i cut it out and put it on top of the ground glass ...

    having a grid is helpful because it shows you how things are leaning and if everything is level.
    a post bubble / level is cheap as dirt as well, and come in handy to put on the back of the camera
    to assure that the film back is not tilted funny, which sometimes is not easy to detect ...
    then the front ...

    and sometimes when all else fails, taking a few steps back and setting up again,
    giving the subject a little more room to breathe helps too

    good luck !
    john

  9. #9
    lxdude's Avatar
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    Steve, just don't have a few drinks before you go out next time.
    I do use a digital device in my photographic pursuits when necessary.
    When someone rags on me for using film, I use a middle digit, upraised.

  10. #10
    Sirius Glass's Avatar
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    Quote Originally Posted by lxdude View Post
    Steve, just don't have a few drinks before you go out next time.
    I wanted to correct the convergence, not over correct it. I have seen too many others proudly show their LF work with the top of the buildings expanding to be larger than the base.

    Back to working on it again.

    Thanks,
    Steve
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

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