Avoiding distortion in Photomerge

Discussion in 'Digital Darkroom' started by vale_surfer, Apr 25, 2014.

  1. Hi,
    The attached pic was taken with a Canon 24-105 L @ 40 mm.
    I took 3 pictures of this factory for a client and stitched them in Photomerge in PS.
    How do i get rid of the distortion in the panorama?
    I've tried the Lens correction filter and straighten ruler in PS but it doesn't work.
    I didn't want to shoot any further away or wider as that would have resulted in even more distortion.
  2. "How do i get rid of the distortion in the panorama?"
    If I'm understanding the problem correctly, the distortion in your sample photo is an inherent attribute of your acquisition technique and there's no way to get rid of it.

    If you want to see the building as if you're looking at it head-on, then the technique to use is Linear Panorama where you walk in a strait line along the front of the building and take pictures at an interval (depending on your lens) that represents about a 50% overlap, then stitched in software.
  3. The distortion in your image is a consequence of the projection chosen for the output (cylindrical), and the large angle of view of the panorama. In that projection, the only straight line features that are preserved are verticals and the horizon. All other straight line features become curved. To get all straight line features straight, you need to select the rectilinear (planar) projection for the output. However, the angle of view is then limited to around 120 degrees, above which you get an unacceptable degree of stretching at the corners and edges. There's also a general "bow tie" effect on the general image outline. I converted the image to rectilinear format using PTGui and limited the angle of view somewhat to give a decent result (some cloning of the sky was necessary to counter the bow tie effect):
  4. Anyone know a stitching program that can do a 360 degree rectlinear ?

  5. Thanks Michael and John
    I'm not able to get a licensed version of PTGUI. I tried the trial version but could not find the rectilinear projection that John mentioned.
    Btw, I chose the 'Auto' option in PS Photomerge
    Will any of this help while actual shooting:
    1. Shoot from a higher vantage point, if possible
    2. Shoot in portrait (vertical)
    3. Shoot more frames and overlap by 50 %
    0r 4. as Michael suggested, shoot at intervals along the building.
    I guess 40 mm wide should otherwise ensure that there's no distortion?
    Thanks a lot.
  6. Q1. No
    Q2. No
    Q3. No
    The angle of view is the problem, and that is determined solely by the position of the camera. Moving the camera further away is the solution, if possible.
    Q4. Linear panoramas only work for flat subjects. You could shoot the multiple images from inside the boundary fence, but that would give you a very boring image, and there's still the sticky out bit at the entrance to cope with.
    In PTGui, you can choose the output projection in three ways:
    1. Switch into Advanced mode (button to the right of the Project Assistant) and select the Panorama Settings tab.
    2. On the Panorama Editor window, click on the Projection menu.
    3. On the Panorama Editor window, click one of the projection icons (sphere, cylinder etc).
    However, in Photomerge, instead of selecting the Auto format, you should select the Perspective option - which shows the familiar bow tie thumbnail.
    Another stitcher you can try is the free Microsoft ICE, which will give you just the same result.
  7. Thanks again, John.
    A few of my photographer friends have shot interiors for hotels and real estate (very well known and popular leisure hotels) and I have seen similar distortion in their pictures. Their clients, however, seem to be ok with this and have put up these pics on their websites and catalogs. My client is a little more demanding and has asked me for a repeat shoot :)
    I guess I'll try all options and then see what works in post processing.
  8. About that word "distortion": what we are seeing in your original image, Vale, and what you have seen in your photographer friends' pictures, is not distortion. It is true perpective.<br>The rectilinear versions may look more natural, but aren't. They are in fact true distortions.<br>Because they look better, the rectilinear version will be preferable. But if the word distortion must be used, it should be applied to the thing that indeed does show distortion.
  9. The only way I have been able to do perfect distortion free panoramas are with a tilt shift lens. Shift left and right while taking 2-3 photos and PS stitching will be the only thing you need to do in post process.
    I don't think I can live without my tilt shift nowadays... It's the only reason I'm not hopping on the mirrorless bandwagon.
  10. I have had good luck with photomerge. Here is an inside factory shot produced by merging 4 shots; 2 on top and 2 on the bottom.

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