So the Sigma 20/1.4 do better than Sigma 20/2.0 regarding chromatic aberration? Maybe someday I get the chance to photograph northern lights and stars (living nearby Oslo but the best chances is in Tromsø, in the Northern Norway). The background for my question is that I hate chromatic errors, and that I make videos indoors from time to time. That is why fast lenses are preffered. At the other hand, I read a test that the Sigma has quite severe barrel distortion, not good for architecture out/indoors and light fall off in the corners. Must evaluate the best compromise. After all, my impression is that the Lumix lenses After all is very good.
Yes, both Sigma 20mm lenses have distortion; the f/1.4 lens has less than the f/2 lens (again, check Lenstip).
As Alan points out, most lenses today have distortion. Historically, distortion was bad, since it's hard to correct distortion in a darkroom. It's much more common now, since it's easier to correct digitally, and as Alan also points out, allowing some distortion into the design gives the lens designer one more "lever" to adjust when attempting to reach a given target. Having said that, some people still do not want to buy lenses with distortion. I'm not sure where I am on that spectrum. I do tend to think that correcting distortion should have some kind of negative effect when viewing at 100%. This is why I like CaptureOne as I can control how much distortion correction is used. And also why I am skeptical of lenses that have LOTS of distortion, particularly when it's complex (i.e., moustache distortion).
Back to corner aberrations: I was talking about coma as opposed to chromatic aberrations. Coma causes points of light (i.e. stars) to become larger discs. Also, some lenses produce "wings" around point light sources, and although this is technically not coma, it is often reported as such. Does not matter what you call it - the point is to understand a given lens's tendency to make stars look like, well, something else. Small interesting tidbit: often lenses that have soft corners also have bad coma.
Another tidbit: like vignetting, coma tends, to varying degrees, to go down (i.e., get better) as you stop the lens down. Combining that with the fact that sharpness often goes up as you stop the lens down, you will find that savvy astrophotographers who have those giant, fast UW lenses often shoot them at F2 or F2.2 or some such, instead of wide open, to optimize the optical characteristics of the lens. Less vignetting = more light. Less coma = sharper stars. Stated another way, if you compare the Sigma 20/1.4 stopped down to F2 to the 20/2 wide-open, the faster lens will give better results, even though it is being shot at F2. And, to optimize the 20/2, you should probably shoot at F2.8.
Getting back to distortion, the 20/2 does have a LOT of distortion. The 20/1.4 seems much more reasonable to me. For whatever that's worth.