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A GIMP Guide, but from a Photoshop User
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The same with Darktable and Lightroom.
Darktable is more powerful (a few thinhs missing though, like panos) but it's UI is way different and super complex.
If you haven't already, give RawTherapee a try. I can't compare it to Lightroom, but I find it a lot easier to use than Darktable.