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Lowering in resolution when I correct lens distortion

Jak O'Momo
2013-05-21
2013-06-08
  • Jak O'Momo
    Jak O'Momo
    2013-05-21

    Hi,
    when I correct lens distorion I notice a lowering in resolution, a diffuse blurring. Is it normal? I apply the correction for a Sony DSC-F828. Thanks a lot.

    G.

     
  • Wolf
    Wolf
    2013-05-31

    Hi,

    correcting lens distortion is an interpolation and thus results in slight blurring. You have to compensate with an increased sharpening.
    I myself do NOT correct lens distorsions except for architecture with straight lines and landscape with visible horizon.
    I hope I could help you.

    Best regards
    Wolf

     
  • Jak O'Momo
    Jak O'Momo
    2013-06-05

    Thanks a lot for your answer! Unfortunatly I'm performing architetural photos! I don't understand why when i correct the distortion manually with GIMP I don't have this problem. Why UFRaw can't perform a prospective distortion of the image and not an interpolation?

     
  • Wolf
    Wolf
    2013-06-08

    Hi,

    indeed UFRaw is able to do such a correction of lens distorsion, even by various algorithms (3rd main tab, 3rd sub tab: You have to choose one of the equations and to experiment with the parameters unless your lens is listed and the parameters a added automatically). But this are only the correction equations. I don't know which interpolation algorithm UFRaw uses internally. Basing on the results I guess it's bicubic. In GIMP itself I use "sinc lanczos 3" which produces MUCH sharper results, so sharp that additional sharpening often isn't needed (optionally selective additional sharpening). But an interpolation is an interpolation and it allways attacks the image quality more or less.
    My suggestion is to avoid interpolation as far as possible unless the distorsion becomes too obvious. And I would preferably work with low-distorion lenses or zooms at their optimal focal length setting.

    Best regards
    Wolf