The Sigma 18-50 EX DC open at 2.8 constant is a good lens. At full aperture, it is usable, but taint with some faults normal for that aperture: vigneting, loss of sharpness, chromatism.
Let's see how DxO Optics corrects thoses faults. Notice that the sharpness is strongly enhanced near the corner and clearly less at the photo center. Then, the gain in sharpness is porgressive: its aspect doesn't look artificial.
One more time, many stars appear in the corners, where a few stars were hard to see.
The chromatism isn't totally delete, but it is efficiently diminished.

Canon 350D + Sigma 18-50 at 18 mm open at 2.8 on a tripod. 30 s at 800 ASA.