Originally posted by malenisjaj I hope this is not off topic. I'm thinking to buy used 10-20 4-5.6 for approx. $350. But since I like to do also landscape astrophotography I was thinking also about Samyang 16/2 ($470). Now, I really don't know what to do. There are also reviews about this lens (Sigma) that it lacks sharpness and there's not that big aperture, if you compare it with Samyang f2, but Sigma has 10-20mm zoom which is really good range.
With the 10-20mm you can get a LOT more in the frame than at 16mm. Fuzziness in the corners is to be expected, even with a 16mm you're going to have more movement at the edges then where you're focused which will lead to trailing/blur, so corner issues (largely) become a non-issue.
That said, I'd see if I could find the f/3.5 on sale if I were going for astrophotography over the 4-5.6. If you're zoomed in for a shot, f/2 is leaps and bounds more preferable over f/5.6 for shooting at night.
In your case, when dealing with just those two lenses, with what you intend to use them for, I'd go for the Samyang over the Sigma in a heartbeat. Considering I almost always argue "Go for the Sigma", that's saying a lot.