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Recently, I started having fun with macro and micro photography by inverting a 24-85mm lens or by combining an inverted 28mm lens with a 70-200mm lens. I shoot in direct sunlight, and the results are good, but not as good as I'd like.

First of all, it should be clear, I don't want to buy a macro lens at the moment. Having said that to avoid obvious answers, I would really love to know how to correct distortions in post-processing.

While I found out that chromatic aberrations can easily be corrected, the results with distortion leave me a bit perplexed.

How can I correctly set an inverted lens profile, so that it does its job?

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You do not describe or provide samples to illustrate the distortion, nor do you give sufficient information to identify the specific lenses you are using. However, it is likely that you are seeing perspective distortion. If that is the case, it is a natural feature of macro photography that is a consequence of the extremely close distance at which you are photographing the subject. It is not a lens defect, and there is little you can do to "correct" it except to move farther away from the subject.

I tried doing what you describe with lenses of various focal lengths. Regardless of which lens I tried (28mm, 35mm, 50mm, 18-55mm, 18-135mm), I get virtually no optical distortion when they are reversed. You can confirm whether there is any distortion by photographing a flat grid.

Although the lenses available to me did not appear to show significant distortion, your lenses may indeed have distortion. Regardless of what software you are using, there is likely no correct way to set lens profiles for inverted lenses or combinations of lenses, some of which may or may not be inverted, because it is not the way the lenses were designed to be used.

However, you do have some options:

  • If the software you are using allows you to manually enter parameters, you may find settings you like by trial and error.

    This is the approach I would take, especially if eyeballing the correction is good enough. Your software may allow you to save presets or templates in which distortion correction settings have already been entered.

  • If your software uses the lensfun database, you may attempt to calibrate the lenses yourself for local use. Do not submit your results to the database upstream because that will cause problems for everyone else.

    Calibration is a bit involved, and you may have trouble finding where to put the settings for local use. The database format was changed a while back, but even recently, not all software has been updated to use the new format.

  • You may attempt to individually correct images in Hugin. It is possible to save a pto file and call Hugin to run on different files. The exact command and how to script it depends on the operating system you are using. It will look something like this:

    nona -v -g -m TIFF -z LZW template.pto -o output input.jpg
    

If you have an unusual level of difficulty, you may consider whether you have "mustache" distortion.

  • Well, yes I was interested more in a general answer rather than a specific one. I find it REALLY strange, that you have no distortion (yes, I'm talking about the perspective one) on inverted lenses, since this is physics, and each lens distorts, however small the distortion may be. I was wondering whether by reversing a lens, the distortion could have been the reverse one, or by combining two lenses, the distortion would just be the sum. Thanks for the last two bullet points, I will definitely try those – Noldor130884 Aug 6 '18 at 11:00
  • When I reverse a single lens and view a grid on an LCD, I don't see any pincushion or barrel distortion. If there is any, it is smaller than I am able to detect, so not something I would personally worry about. – xiota Aug 6 '18 at 11:26
  • As for so-called perspective distortion, that is a consequence of the extremely close distance at which you are photographing the subject. It is not a lens defect. The easiest way to "correct" it is to move farther away from the subject. – xiota Aug 6 '18 at 11:27
  • My bad, I thought that perspective distortion was the pincushion of barrel one. That was what I meant anyway. – Noldor130884 Aug 7 '18 at 5:10

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