Sigma 20mm 1.8 on FX
I suggest you use a good post processing lens corrector when using this lens.
I have had that lens, I named it as my "night landscape lens" as it is used almost exclusively for that application when I want starry night without star trails all at a usable ISO sensibility.
I use only raw on DX format but even at cropped format is was not a great performer wide open (well, if you buy such animal certainly you want to work wide ) but when DxO created the profile for this lens for my D300S I got an important improvement. Today I am very satisfied with its performance.
I guess you would have much better results when using a raw developer as DxO. It is not a matter of optics only - the post processing certainly counts.
O.Cristo - An Amateur Photographer
Opinions of men are almost as various as their faces - so many men so many minds. B. Franklin